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.
This doesn't make sense for a tank vent. A vent would be from the top of the tank. A proper vent could not siphon as it should be air at the top of the tank. The only time a vent would spill water is if the water level in the tank was above the vent. This is possible if the external tube of the vent is not high enough to deal with the coach leaning to that side. Never plug up a vent, that is asking for a problem.
I had to modify mine as the vent tube was only an inch above the top of the tank and would spill water on every left turn. Added 12" of vertical tubing and haven't had a problem since.