I listened to the "get as big as you can" wisdom when we got our first TH a 39' fiver. Now have a 24' TT/TH. I would consider even smaller. We spend very little time in ours when camping. The only thing that is not as nice is sleeping on the Happijac when the two motorcycles are loaded. However we only do that on rare occasions on in route over nighters. Personally I think maybe one size bigger than what it takes to do the job (as long as the bathroom is big enough). With a TT you can put one motorcycle or ATV in the truck.
If you don't go too large you are not stuck driving a heavy truck the rest of the time. Bonus!