It could be the water valve usually located in back. When you flush, put your fingers back there and see if you feel water leaking out. If you have a mirror small enough- see if you can see it. Google rv toilets and see if it's a Thetford, SeaLand, or something looks like it. You could also see if someone chimes in who has a '77 Hitchhiker too. You'll find out what's up...just like I did.