C'mon guys . . there's at least two, and probably three threads up here that deal with overheating, and they all "boil" down to a couple of things, either the water isn't in the radiator long enough or the radiator isn't big enough. A third might be not getting enough air through it in the right places.
On th' Mopar Motor Home. If it works good at 55 but heats at anything over, it's one of two of the above . . It ain't rocket science.