His Generator Does Start! So that means that there is plenty of usable Gas! Seafoam will certainly help ,in any event!
All of the suggestions are good except it is Not that easy to cut a Hole in the right spot . Better to drop the Tank.
One must presume that the Mechanic has done due diligence ,but there are other components that can cause the Fuel Pump to stop running.
These include the ECM often referred to as the Ford Brain Box which hasn't been mentioned here.
Other parts aside ,from what I have been told, many In Tank , Ford Fuel Pumps have been replaced in those 1990's era Fords with External Holley Pumps accompanied by a Spin On Fuel Filter.These we an option offered by some Chassis makers.
We had one on Our Old Southwind !