1989 Chevy Truck engine losing power

My question is about a winnebago rv on a p30 chevy chassis, 1989 with 454 engine and carburater.

The rv will run great. It starts easy and has lots of power. After driving it for 50 +/- miles it will just start to die as if it was not getting enough fuel. You can pump on the gas pedal and it will still just die. I wait for a while and it will start fine, but a while later it does the same thing again. We have done the following: -changed the fuel filters under chassis and in carburater
-changed the fuel pump on the engine
-taken off the fuel tank and voila, another fuel pump in the tank. It was all burned out from "maybe overheating"? We changed that. It still does'nt work! Is there a relay for that pump &/or does it even need that pump? Help. Any other ideas? (This is the short version of all of the things we have tried but we feel it is lack of furl related.)
Thanks