RE: 1997 Holiday rambler Endeavor LE Ford 460
Start with known problems...i.e., the brake system. Raise each wheel and apply brakes and check if the calipers release proper. With those few miles I would think there are problems with the seals sticking in the calipers causing extreme resistance and slowing down the vehicle. I repaired a gentleman's Jaguar that was doing similar to what you described, and it was his brake power booster activating the brakes under acceleration and speed. Check brake drums/caliphers temps and drive it and if it slows down after a few miles check the temp at the time to see if their temp has increased a lot.
Is the engine fuel injection or carb? Check for any codes showing. Does the engine accelerate proper sitting in park? Is the transmission slipping? Has its fluid and filter been changed recently?
