Sorry I haven’t replied sooner but wifi here is a bit sketchy. First a huge THANK YOU! for all your replies. They are very much appreciated.
Secondly, some answers to your questions. The unit was stored full of RV anti-freeze that included the toilet and all the waste valves were closed. The seal on the toilet seems to be solid as there is no change in the water level over night.
We stayed outside Venice, FL for one night before coming to Sanibel. We did hook up the water but didn’t hook up the waste line. We’ve just now discovered that that RV site admits to having red ants. I didn’t see any while we were there but perhaps they found us. Two nights later the ants were in the toilet.
I like the black tank idea but can’t see how they got inside the toilet. This morning I found an ant in a glass that I was filling with water. Earlier we found one in the bathroom sink. This would seem to suggest that they got into the main water line but I have no idea how that could happen.
Looking forward to hearing your thoughts.
Happy New Year!