welcome to the forum.
Trucks don't tow trailers determined by the trailers length nor does length determine a trailer weight.
Generally a 37-40' 5th wheel trailer for full timing will have multi slide outs and be very heavy....so its decision time for you to decide on the floorplan you want.
Then once the trailers GVWR and dry weight gross weight is determined we can help you with a tow truck.
The one ton DRW trucks will handle GN/5th wheel trailers 30000+ lbs with the right engine/tranny/gearing package.
A 3/4 ton truck with around 2800-3200 lb payload in the bed is out of the running for a heavy 40' 5th wheel trailer. And on the other side you don't need a class 8 tractor to pull that size trailer.....unless you just want one.
A one ton SRW can have up to 3800 lb payload in the bed before exceeding axle/tire/wheel/rear spring pack ratings
And of course a one ton DRW can have up to 6500+/- lb payload in the bed. Good hunting