Problem with starting

This is what has been changed and then i'll tell you what it's doing. Spark plug and wires have been changed, all filters are new, the main gas pump has been changed, the rear tank gas pump was changed thinking it might be the solutions.

This is whats going on: If I switch to my tanks to the rear one (because I have 2 gas tanks. Its a class B rv) and lets say it starts which it seems not to theses days, when I shut the engine overnight it wont start the next morning. Starter works perfectly but wont start the engine. If it does, its like gas is not pumping and it can staul but at one point itll work if the gas goes in. So we changed the rear pump thinking it was the problem and even the main gas pump but guess what, it did the same this morning. My mechanic which I like and trust seems stumped and that makes me nervous cause I need my rv asap to leave for Toronto or I loose a lot of money. Now 1st we changed the main gas pump, now the rear tank gas pump and it even does it a bit (wont start right away) when switched on the front gas tank ! So what is it ! Please answer asap it'd be appreciated.