Faulty fuel pump or carburetor

We enjoyed our motor-home for the past five years. The speedometer shows 165,000 km, it functions very well and gives us great pleasure.
Unfortunately the engine stopped on the highway a week ago and we had to be towed. The mechanic at the garage found and repaired the lack of current to the gas pump. We got back home from there without a problem until the next morning when we tried to re-start the vehicle; it started but the engine did not function until we poured a couple a table spoons of gasoline in the carburetor, and it stayed that way every time we want to start the vehicle. The gas pump is functioning good and shows a pressure of 20 psi. At start, but twelve hours after the vehicle was stopped the pressure dropped to 10 psi. We checked all the fuses, all okay.

This is a Chevrolet 1993, 7.4 litres, 454, with a registration #1GBJP37NOT3307605.

The gas reaches the carburetor without problem, but it appears that a last valve on the carburetor does not open until the engine starts. Could it be a relay problem?
If not what could it be? Could you help us resolve this problem? We would really appreciated it.
Thank you very much for the attention given to our request.
Constant Guay