What I did to our Itasca which sounds like it's plumbed the same as your mh.
I threaded a piece of 3/4" CPVC pipe for a garden hose, I did this with my lathe, then solvent welded/glued it to the end of a CPVC Tee.
I put a plug in the other end of the Tee, and drilled and taped it for 1/4"-20, and took a 1/4-20 threaded stainless rod to screw it through the Tee, so it pushes the spring load check valve that is just inside the mh, attached to female hose fitting on the outside of the mh. You need to make a guide to hold the rod on center.
You need to be very carful not to push the check valve open very far.
You need to point the side opening of the Tee up and add to it so you can pour water into it.
What I did was add a garden hose valve to the side, of the Tee, which is pointing up, and added a hose valve. Then a clear plastic hose, which goes to the water bag, which I hang from the top of the motor home.
Dusty