My trailer is nothing special, but you asked about packing gear as well, so here goes:
I've been using a 101" x 10' snowmobile trailer to haul bikes for years now. Snowmobile trailers have some specific pros and cons (I can elaborate if you'd like), but they definately offer deck space. They're also among the lightest trailers available for their size; I think this one is around 450lbs empty.
Four or five bikes is the norm for riding trips (usually three of us traveling together):
It looks like a hodge-podge packing job, and it sort of is, but in reality each thing you see on the trailer has a semi-specific position that has been refined over years of trips. It's usually the same two or three of us going on these trips, and things have become so habitual that we load up surprisingly quick without much need to talk to one another much at all anymore. Well, I guess usually the placement of the last couple of tie-down straps does tend to get over-discussed.
Probably the most specific postioning visible in the photos is the fuel jugs. One of the main reasons those are in the rear is so that any spills or leaks don't get into the other gear. Also, having them all lined up at the back like that simplifies fill-ups at the gas station. We usually try to haul them empty most of the way to our destination, stopping to fill them at the last gas station that offers non-oxygenated gas from a dedicated hose.
Hauling the jugs empty also allows them to be filled and used as balast along the way, repositioning them if necessary. On one trip years ago, we picked up an additional guy and his machine & gear along the way, and struggled to get things loaded with enough tongue weight to make the trailer tow worth a shit. Filling all of the gas jugs we had and strapping them as far forward as possible helped.
We've also had too much tongue weight when my buddy "tops off" the ice in that huge cooler of his pictured on the tongue. Filling the fuel jugs on the rear somewhat helped in that instance as well (not ideal, but better than nothing). The fuel jugs pictured hold 16 gallons, which works out to just over 100 lbs. The water jug is another 50 lbs, but that's usually filled at home (for water quality/taste reasons).
One other thing that I usually put on the back of the trailer that isn't pictured here is an empty (or mostly empty) plastic tote. I prefer to buy most of my groceries en route, prefferably as close to our destination as possible. We've found this to be the overall quickest way for 3+ guys to coordinate and split food for the trip, and it's one less thing to deal with packing at home (our wives get annoyed with any advance preprations, so avoiding a grocery run a night or two before departure helps keep the peace at home--YMMV).
Also, and perhaps more importantly, I like to buy my groceries at the closest well-stocked grocery store to our destination. It's an "ambassador of the sport" thing for me, not only spending money in the towns near where we camp and ride, but most importantly you're towing bikes into their town and it's flat obvious that you're spending money in their town. We tend to be overly polite while in town, trying to make the best impression possible. Also, we do any shopping for adult beverages in our hometowns before we depart, again to keep up appearances and represent the sport in a positive way.
I've gotten off track here, but my point was simply that the empty tote at the back of the trailer makes it easy to load the groceries right into something without having to partially repack a bunch of stuff just to find a place for it. It also keeps things like bread, buns, and chips from getting smashed.
Looking closely, you may or may not be able to see cots, a canopy, plastic totes, a loading ramp made from an 8' 2"x12" (with a long strip of coarse sandpaper glued to it), a rake etc. It's pretty easy to slide most of that stuff around when loading/unloading bikes, and things like totes and coolers usually never come off that trailer for the whole trip. Once at camp, fuel cans get moved to the shade underneath the trailer.
Of what you can see that I wouldn't recommend, definately not the green Rubbermaid tote in the center photo. We've since been upgrading our totes to more robust water & wind resistant types with latching lids. The big gray one in the top photo is my buddy's. It's lid latches very securely and stuff inside stays dry, but two totes half that size are easier to deal with.
Also, don't haul the grill on the roof like that. It gets full of bugs that later get baked on, and my buddy claims it costs him 1mpg to have it up there. I can't confirm that, but I can confirm that's a huge pain in the ass to get it up and down from there without scratching anything.
Some things I make it a point not to haul on the trailer. Clothing bags, sleeping bags, tents, and other things that need to stay dry (and are easy to stuff into the two vehicle) are obvious, but I also don't haul toolboxes on the trailer anymore. The trailer just doesn't ride as smoothly as the two vehicle, and over time the tools get kind of rough looking from banging together and vibrating along on the trailer. Similar are camp stoves, grills, lanterns, and things like that that don't tolerate rattling and harsh bumps particularly well.
We used to haul with a 20-lb propane cylinder, but that proved to be enough of a pain that we've gone back to the little disposable ones, which are also a pain in different ways.
I breifly mentioned adult beverages above, and that's another thing that factors into our packing system. Two of us that go don't drink beer at all, and it turns out beer is not a very efficient way to pack and transport alcohol. Instead, we usually have a couple bottles of whiskey, brandy, or maybe some rum. That stuff comes in very robust glass bottles, doesn't need to be kept cool, and is usually pretty easy to stuff in somewhere. If we bring soda for mixers, 2L bottles are similarly simple to pack. We've picked on my poor cousin (the third guy who usually goes with us) so much over the years that he hardly dares bring even a 12-pack anymore. If anything, he'll bring just a few bottles of what he claims to be "really good" beer these days, so we figure we're doing him a favour.
Hopefully something in all that is helpful to you. I think back to some of my first riding trips with this trailer (and its predecessor), and it's funny how much of a mess we had. Things now wouldn't look much different to an outside eye, but the little things you learn from doing things wrong over the years really contributes to eventually developing a system that works for you.
I recently bought a Jumping Jack tent trailer
, so I'm kind of back at square one with my loading system. I'm very exicited though, as the Jumping Jack will make several things so much easier, and it pulls much nicer than this trailer does. Loading and unloading bikes is proving to be a challenge, though.