Stan
All depending on how much you visit your dad it wouldn't be that hard to locate where the pipe comes through the wall of the house and install a clean out...
Before I would unhook my camper from shore power hook up to my truck and drag it down the road to dump my tanks, I would do what Matherne said a few posts back and clean the bathroom...
Who knows your dad might even appreciate it...
IMHO as an old Marine I have cleaned many a bathroom (head)...