OP, not the same setup but similar experience. We have a Coachmen 28QB that is literally 32' bumper to bumper. It's powered by a 6.0 V8, We currently tow a dolly/car combo weighing about 3800lbs.
On our recent adventure out West, we encountered several long "hills" on the interstates that had us running 40-50mph! (We normally try to run about 65.) As someone noted, I had to learn to enjoy the journey. LOL It still bothers me to run that slow but, I realize that is reality for this type of vehicle.
Good luck,
Gary