The only problem I've had with my gas engines is the fact that I always seem to have too small of one for my needs. My 89 had a nice running 302 and did a decent job pulling the camper, until I took it to Branson. I was lucky to get to the top of those hills doing 30 mph.
However, the GTG van is another story. Those same hills don't really seem to faze that old girl. It's a 351W. You'd think it was a 460, the way it pulls. And it feels like a 460 when I pull up to a gas pump. Unloaded seems to be a consistent 12-13 mpg. Pulling the camper it drops to 8. That's probably why it's got a 36 gallon tank.