If he doesn't know what hes looking at its even harder to diagnose.
as far as a stock
LP goes, i have seen several stock pumps that make only 7-10 psi @ idle and never hold more than 5 psi under any load. even mostly stock.
Todd will tell you he doesn't want any more than 10 psi coming into a street pump. more is not better, the pump just needs 5 or 10 psi so that its not having to ask for the fuel or pull the fuel in.
rail pressure is a good indicator as stated above. you need to watch RP as your driving under a load.
You only have a 16' trailer.... that truck shouldn't even notice it much. how does the truck act unloaded when you get on it?
quit guessing and get a gauge on that
LP!!!!