One gallon of rotten gas is not going to cause a truck with a near full tank to refuse to start unless, as Caddyman mentions, that gallon was water, etc.
There’s still not enough info to even make a wild guess. The only reason I mention the fuel pressure sensor is because they have been known to be a problem now and then.
The sensor replaces the former check valve in the fuel pump which maintains residual fuel pressure when the engine is not running. The sensor acts up for whatever reason and no fuel is going to get past it and to the injectors.
The mechanic has had the truck since last Tuesday and by last Tuesday he should have at least been able to give you a vague idea of what the problem is and what’s missing. (Missing meaning lack of fuel, lack of spark, etc.)