Fuel cap
Try to loosen the fuel cap and test drive he truck. When I purchased my D/A RV it had a locking gas cap on it for a gas fuel tank (unvented) and the RV had problems much like you are having. Additionally, I agree with Jim clean the fuel cooler and never run less than half a tank of fuel in hot weather. If you run less than half a tank of fuel on a hot day is likely to bring up a P0087 code.
Good luck
John
*2006 Chevy, 3500, 4X4, DRW, (LBZ) D/A, CC, LT, 252K Miles, 19.5" Wheels, Mag Hytec Transmission Pan and Differential Cover, SS Grill Guard, Racor 2 micron aux fuel filter, 100 Gallon Aux Fuel Tank, using Edge Evolution, Predator Diablosport, Kennedy ECM tune, Fitch Fuel Catalyst.
*2006 Four Winds, Dutchman, 36', RV, D/A, 5500 Kodiak.
*1993 Harley Davidson, Turbo charged, Springer Softail.
*2007 Pontiac Solstice.
*2005 Jeep, Liberty, CRD (diesel).
*Full-timing in USA, see America first.
BUY AMERICAN or CANADIAN, NOTHING from CHINA .