I would install a tachometer in the truck if it doesn't have one. You will probably be surprised that 3500 rpm seems awfully fast if you've been driving conservatively, without a tach.

The motor will have to work awful hard to move a heavy truck and camper up long grades, but if you keep it in the 3000-3500 rpm range it should be fine at WOT for the duration. A 351W in a boat will do that for hours on end.

Oh, you don't need one of those 8000 or 10,000 rpm ones. A good old Sun 6000 rpm tach is quite sufficient, and cheap at swap meets because most people are unrealistic about the engine speeds they will be running.

Do you have an automatic or manual transmission?