C&J wrote:
Ah, I should have said that the overflow hose is located INSIDE the tank, with just an inch or two exposed at the bottom of the tank. The RV shop said we could put a vent on top of the tank--except they can't get at the top without major surgery.
That is surprising. All of the many gas Winnebago's I have seen and the ones being built on several factory tours have a fitting spin welded to the top of the tank. The overflow tube connects to that fitting and goes down below the bottom of the compartment so the overflow goes on the ground.
Usually there is a plastic insert with small holes that keeps bugs out of the tube. Nothing protrudes into the tank. On mine when I complained about the water loss on turns, the factory added a loop that went about six inches above the top of the tank. That stopped the loss.