Should be pretty simple to dig down a foot or so and saw off the 2" abs/pvc cleanout pipe, then glue a 2"x 4" abs/pvc reducer onto the 2" pipe and then glue on a short piece of 4" pipe, then a 4" abs/pvc threaded female cleanout fitting so it ends up flush with the ground......then you can screw your threaded RV sewer hose hookup elbow into the 4" abs/pvc cleanout fitting just like you do in an rv park.....put a 4" abs/pvc plug in it when you are not using it....thats the way I would do it anyway....abs/pvc is really easy to work with and fittings are cheap & easy to find at any hardware store.....