I've run into a number of weird sewer ports in campgrounds around the country. To connect to these I have made several adapters and purchased a couple others, in addition to the rubber donut.
The top-left is a Valterra adapter that fits a three inch threaded port. Top-center is a couple of three inch ABS fittings I put together to solve the low port issue, but only works with a three inch port. Top-right is a three inch female to female threaded adapter I made for additional rise that I needed somewhere that I don't remember now. And the bottom fitting I made from a Valterra part, the red threaded part, and an ABS fitting that I cut from a three inch adapter to convert to a four inch port. Below is a Valterra clear bayonet elbow.
Threaded Mode
