Been there, doing that. A standard production CT can last decades, 24" c, floor bracing is fine for a bike and PPC with a 3/4 ply floor. Mine is a standard "Razorback" trailer built in Arkansas. I bought mine used and at a steal of a deal, like new for $2,000.00 and I got it just to haul my 1800 STX trike, no camping in mind. Then I caught the bug to use it for camping and hauling, touring the country with my trike. So;
Do not entertain the thought of a 6' wide CT if you do not plan on going on narrow off road trails, really, you don't need to go down rabbit trails.
A 7' wide allows you to have an east west bed, after you insulate and recover the walls. Mine is a 6x14, V nose and floor plans are limited, a 7X14 would be much better.
CT conversions for a nice toy hauler usually use jackknife beds that fold down for a twin bed. These $800.00+ beds are about 30/32 inches wide, two will take up 60/64 inches and that at 72 0r 74 inches long gives you a nice queen bed.
Another option is the elevator bed, queen size, lots of fabrication and DIY work or about $1500.00. This would not give you room to sleep with toys inside.
That bed gives you something you and your beloved can crawl out of while one crawls over the other for any midnight reason. Being older, we like to get up out of bed as we do at home, on the sides, sit up and stand up. To do that you need at least a foot between the beds in the center of the trailer. 7' wide is the minimum to make this happen.
In a 14 or 16 foot X 6 or 7 foot, with a 6, 6.5' ceiling, sleeping with your toys inside might work for a single or twin bed, but it will be cramped. Tossing a bed on the floor while loaded might be easier.
At 7' wide the wheels and fenders will be outside, on wider trailers you'll have wheel wells inside to work around with your floor plan.
Next, look for a trailer with the longest tounge, cargo trailers are built to maximize profits and made with short stubs in front to pull them. The longer the tounge the more control you have for backing, but going too long means a shorter break over angle for rough terrain. Be aware that a V nose extends the box further to the front over the tounge, the distance from the coupler to the axel may be the same as a flat front box trailer.
Single axel is plenty for your stated needs. Single axel means 2 tires, not 4, 2 brakes, not 4, along with half the suspension of 2 axels, a single axel has the ability to turn sharper corners and tip up and over obstacles easier than a dual axel. Stock single axels are rated for 3500 pounds, you might find an optional 5000 pound axel, but for your needs, 3500 should be plenty. Just build the inside light and not with 2x4 lumber.
Leaf springs are more than adequate for mild off road use with a single axel. Everyone under 55 years old should know, us old guys went everywhere you're trying to go, years ago with leaf springs, most often with 2wd. Air bags, torsion axels, even 4x4 simply make up for driver faults by less skilled drivers, so, no, you don't need "state of the art" to enjoy your manly adventures.
Siding attachment really doesn't matter, any real advantages are minor in real life, screwless might look nicer, but the roof is a different matter. Try to get a seamless, one piece roof, they won't leak in 3 years after rock n' rolling.
You didn't ask, but fenders with a flat top are more useful as a table than round fenders.
Kitchen/bath generally goes up front with bunks/beds on the sides that title up or moved to haul toys in the center, midway and a bit to the rear. A 7' wide gives you enough room up front in a flat face or V nose to have a limited kitchen and toilet, maybe a shower, allow at least 3 feet back for counter and standing room.
Framing is adequate on commercial cargo trailers, they all must pass state inspections for the loads anticipated, don't get over hyped on the size of your tubing.
However, a walkable roof might be hard to find on a trailer lot and is very good to have if you can find one. I'm putting solar on mine and it would be very handy to be able to crawl around on the roof for maintenance.
If you buy a new trailer you might knock off a few bucks by getting one without any interior built in, leave the plywood walls on the floor, but with electrical done, just tape fixtures, light switches down. This would allow you to insulate much better than any factory job, IMO. You could also run your own electrical before you wall it up.
My trailer doesn't have brakes, I hauled a 500 pound riding mower once and there was no problem, didn't even feel it back there in my F 150. I'll see about getting brakes installed before I venture out, a single axel trailer can get wild if it ever broke lose or if the tail starts to wag the dog from a flat tire.
I'd say the best size for you, hauling a PPC and a cruiser or adv motorcycle would be a 7wx14lx6.5h. At 7x16 you might have a full time bed, east west.
Bob Wells, Cheap RV Living. com and Bill of I Ride Tiny Homes are on youtube with hundreds of cargo trailer variations. Investigate, but don't get hooked on looking and studying plans for years like I did. Get a trailer, toss your gear in there and go, you can work on interior stuff while on the road.
