I have the same exact problem with a 1987 Ford E350 chassis motorhome that previous owner let sit in a barn for four years. Cranks and fires on starter spray but won't run. Cannot tell if electric fuel pump in the rear tank is working. Diagram shows a tip-over safety relay in cab passenger side by the heater motor but tests show it appears to be working. If anybody posts a solution to this problem, I would love to know the answer also.
jh