You can get car haulers that aren't decked out to be a "race trailer". IE you can get them with plain unfinished interior and without cabinets and junk. I'd consider that here. They'll typically be 8.5ft wide (102in total overall exterior width) and around 81-82in clear between the fenders.
OK my thoughts on dirt bikes and/or ATVs
* Standard interior height is usually 6ft-6 or so. Most places will make them a bit taller as an option. More height = more towing surface area. Don't go taller than you really need to. As mentioned, remember to consider the torsion spring for clearance purposes.
* You'll probably want to strap miscellaneous things to the walls. Make sure to get 16in stud spacing. Standard is usually 24in.
* E-track or L-track on/in the floor (either surface mount or recessed mount based on your preference). These offer an abundance of tie down options.
* Extended tongue. This will help keep from banging your camper on the corner of the trailer in sharp turns.
* I've found that v-nose doesn't really tow any better, but they do have a little benefit of extra storage room. They do present additional surface area to cross winds. V-nose tend to make it more difficult to attach a weight distributing hitch, and precludes you from running a toolbox on the tongue, if that's your thing. I prefer a standard box.
* Don't automatically get heavier axles. Figure what you'll actually haul and try to stay around 80% of the axle ratings. They ride much smoother that way. This is especially true for torsion axles. 5200lb axles on a 16ft trailer is likely overkill, but maybe not as much on a 20ft.
* Bogey wheels. These go by different names depending on who you talk to but they're little steel rollers positioned near the rear corners. Helps to keep from dragging the trailer frame when you inadvertently high-center the thing. This happens more often than you might think, especially on longer trailers. I'm really surprised they're not more common.
* Interior lighting. You can go as basic or advanced as you like, but it makes load/unload in the dark so much nicer.
* Power tongue jack. Your shoulder will thank you.
* Consider a trailer that's less than 10k GVWR. Many states (TX included) require a CDL if your truck/trailer combination GVWR is over 26k. A 14k trailer will probably put you over that limit with your dually. This is not tied to your loaded weights, but by the GVWR stickers on your rig(s).
* If you have the option, get the thicker exterior sheet metal. They're harder to ding and don't warp nearly as bad in the heat.
* If you're carrying fuel, make sure to have appropriate venting installed, if it doesn't come from the factory that way. Gas fumes are heavier than air. So your intakes will generally be front-top, and exhaust will be rear-bottom.
Trailer prices are through the roof right now, assuming you can even find one. Good luck with that part.
