We own another brand, but I know you Airstreamers know what you are talking about when it comes to RVs. Since the holding tanks and toilets are all basically the same, the air vent from the black tank might be my problem. A dirt dobber might have built a nest in there.
The splashing is caused when there is water in the toilet bowl. When I flush it, the air from inside the holding tank pushes back into the rv causing a "small eruption"!!!
Thanks for your input. Any other suggestions would be appreciated.