1988 Other Ford Models fuel pump pressure

Please provide specs for fuel pressure for a Ford E 350 motorhome with a 7.5 liter engine. We are only showing 35- 40 psi and this seems to be low for a high presssure pump. We have no power uphills and seems to be dumping gas as it only got 3 mpg as we nursed it home. We installed new plugs, wires, cap, rotor, map sensor, and O2 sensor. Cat conv. Ok. Have not pulled exhaust yet as pressure seems strong and unit is welded. Vacuum pressure seems to be weak. Any suggestions much appreciated.