Couple of comments re Tank bags.
I had a DriRider bag that had a good size clear top to put maps and a4 route sheets in. The zip ran down the long sides ...not across the top or bottom. That's makes getting maps in or out just that bit harder. So look for side zips or zips that open 3/4 way round.
The DriRider also had an expandable section...at the top of the bag not the bottom. With expansion from the top you get a more stable bag as the body has not been compromised.
The same bag had a small pocket across the front and two side pockets great for your phone or wallet or lollies.The zips on all these pockets had strong weather fold on them.
That left the main bag for cameras, spare gloves,your liner or wets.
Now that bag just plain wore out. I got well over 80,000K's out of it in all weather and never had any wet gear. I have not found another yet.
I replaced it with a MotorDry number. Its a two part thing. Big map holder ...but its in the bottom section. The bulk of the bag zips to it. Sounds fine but it leaks like the labor party.
The top map pocket has an opening too narrow to put A4 or maps in without rolling them first. And you need your gloves off to work it.
The two large side pockets hold heaps but the zips are not covered so everything gets wet with a little rain.
The bag is expandable but from the bottom. Net result is the thing has no stability in the body and rocks from side to side like a drunk.
Might be points worth a look at as you shop.