I have a 05 duramax, goes into limp mode with codep1093 with camper on, changed fuel filter 3 times, each time I changed the fuel filter the check engine light would go out after starting. took it in and they said Fuel pressure low, pressure regulater in pump is sticking, balance rates high #45&8 injecters, pump does not always build enough pressure. Any suggestions on where to start. Could it be something more simple? I know what dealers like to do! Is ther a pres reg in the pump?