Glad to have you here with us.
Sounds like your MH was never de-winterized. It would seem like the low water drain is open and that is what you are plugging with your finger. If the heater by-pass is open you will be putting the water into the hot water tank. You also need to close the hot water drain. There should be a valve for both of these. See if that solves your problem.
Good luck and Happy Trails!!!!
Tony & Ruth........... FMCA#F416727
Newmar Dutch Star 4320, Spartan MM Chassis, Cat C9, Jeep Grand Cherokee,with Hemi, hooked up with a Blue Ox Aventa LX, and Brake Buddy.