The 2500's have a pretty convoluted fuel system. Actually have 2 tanks although there is no manual switching between them, they function as a single tank. Fill goes to rear (secondary) tank where it is transferred by secondary fuel unit to front (primary) tank. If you are able to utilize the entire 38.5 gallon volume of the system you probably need the front (primary) pump.
Have a leak somewhere in mine and have not had time to crawl under it to find it (doesn't help that it seems to be either raining or even snowing when I have some time to inspect it).