If plumbing & tank similar to my Class C, hopefully the cure is under $2; mine had a cracked 1/2" MPT x 1/2" Barb PLASTIC ELBOW for VENT that some PO had piled PUTTY around instead of replacing. I took the opportunity to replace moldy vent hose and Fill hose (1-3/8"ID fits the 1-1/4" OD fill Barb, and I accessed in top of compartment containing water pump; THE other end of hoses accessed by removing fill port box. BUT, this would NOT explain the leak w/ PUMP running, so you may have a 2nd issue? Most FW tanks have no plumbing on top, but is rather on top of the END of tank, and BOTTOM of End gravity feed to pump? GOOD LUCK P.S. WHERE is your pump?
(TerryH.) 2000-GS Conquest Limited 6266 Class-C 99-E450SD V10