Check to make sure the vent hose out of the top of the tank has not fallen below the main fill hose.
The vent hose needs to be above the top of the fuel tank so fuel does not get in that hose.
On our new to us Workhorse, PO had had the fuel tank replaced. The installers didn’t properly strap the vent or fill hoses back up to the chassis correctly - made filling the tank impossible without filling slowly and at a cock-eyed position of the fuel handle.
A simple repair with a couple of ty-wraps.
But fuel being in that hose will prevent the tank from venting fuel pressure buildup that occurs in any fuel tank as the coach is driven. When the cap is removed, the pressure attempts escaping thru eithe the fill hose or the vent hose, and if there is liquid in either one of those hoses, you will get splashed.