My dad used to have an identical style camper on his '88 F250. It had a 400 Modified and it was a dog. Wound up putting extra cooling on the engine and tranny to keep from burning it up. But to it's credit, it never broke down and he made several long trips (Texas to D.C., Texas to SoCal, Texas to Minn.), just never got in a hurry.
Now that I think about it, he probably had 5,000 lbs of crap thrown in that camper.