I Spent Lots of Time on This
I have a motorhome and seriously thought of trailering in an enclosed trailer. After much research and effort I decided that the time/effort and frustrations were not worth it.
I wanted a single AXLE trailer. It's almost impossible to find a trailer large enough to haul my BMW GS Adv with DMC sidecar (wide one) without going to a custom trailer manufacturer. Simply said, select a Haulmark, etc. and you will have to go with a dual axle trailer.
Then you get to the ramp and the width you lose if the trailer has any LIP around the door/entrance/ramp. No matter what, it's going to be close. The weight of course will demand trailer brakes to be legal and while inertia brakes seem to be ok for boat trailers apparently a no-no I was told for non-boat trailers. Can't verify this. But, trailer brakes needed and of course then a brake controller in tow vehicle.
Some life style changes put a damper on this project for me, but it would be worthwhile to talk to anyone that tows with an enclosed trailer. E.g. Jay or Barry at DMC passed along some info that pretty much put the shutdown on trailering. It just ain't that easy as no matter what, it's a tight fit, and anyone who's towed anything can affirm that trailering is a PITA.