There are also some 40'-45' truck conversions that have upto 30k lb towing capabilities if you want to have an enclosed car trailer. Since these started life as a Class 8 truck (the over road trucks), they have the power for 80k pounds. There are ones with bunk models and lots of storage as well, multiple bathrooms, etc. Look into Dyanmax, Haulmark, etc.
Don't worry on these about needing to shift, the new modern ones are basically an automatic transmission with no clutch pedal.
There's also a guy in Missouri (Hawk Engineering) that makes custom conversions to meet anything you'd like, including the trailer. This is just one of a few I've found that does one-off customs for reasonable prices.
There are a lot of campgrounds (public and private owned) that can handle 65-70' rigs without disconnecting a trailer (or toad), so don't worry about length or having to park a trailer somewhere. When you look into campgrounds just make sure they can handle the length of whatever you end up with.
