I bought a radiator for my 1986 winnebago RV gmc chasis 454 the new radiator has no place for a cap on it but has a hose neck sticking out the front of the RV what is this for?

It goes to an expansion tank. The expansion tank keeps the radiator full by allowing the hot coolant a place to go as it expands. If your old radiator had a cap, they've sold you the wrong one.