Hello. I have I 1989 Winnebago warrior. 454 . Sitting idling,I could rev the engine until it blows up,no problems at all. Once I put it in drive,If I punch the gas,it cuts out. If I find this sweet spot,I can get it up to about 45,where them it will cut out again,but again,I let off the gas and find the sweet spot,I can maintain speed. Also the moment I hit a hill,and little incline,it starts the cutting out again,like it’s getting too much gas. Some backfires if I don’t let off of the gas going up hill.
I actually stalled on a hill,thought it was done. Started it back up.,and went up the hill at 15-20 mph. But can’t go over that without choking out.

 

start with the basics. is this still tbi or carbed? have you check your fuel pressure? how to the injectors look while running