Napa lists a replacement fuel pump for a P-30 chassis that isn't the right one. I used one from Napa and later found that it could not keep enough fuel to the throttle body. I bought a "Carter" pump and solved the problem.
Also, the in-line fuel filter along the underside maybe half way the motorhome could be clogged. It isn't an expensive filter and isn't a big job to change out.
I don't know if your motorhome is a P-30 or not. But, the fuel pump issue may be the same.
Me & my Hun RVing for fun!