One important thing missing for "winterizing" with RV antifreeze is a "Bypass" valve! Your inlet and outlet is there. The inlet is on the bottom and the outlet (going to your sink etc) is on the top.
In other words, when you try to winterize using an onboard waterpump (you DO have a water tank I believe you said?), then it's going to pump into the water heater as well without a bypass valve blocking it.
It looks like the nipple you show in the pic was an add-on by another owner. It ALSO looks like you could see it from the out door portion (where you open the access door) of the water heater???? Can you get us a picture looking at the outside of the water heater with the access door open? I'm just curious.
You say the two hoses are running to the "tank". One of them has to come FROM the fresh water tank, but the top one has to be the supply to your faucets/commode, etc. Otherwise, you'd have a continuous "loop" if both are connected to the "tank".
Here's a picture of my water heater made some time ago when I was getting ready to install the "HottRod". It clearly shows my bypass valve. On the styrofoam insulation, I have it written so I can easily remember which way is bypass or bypass off. If it's perpendicular, it's blocking, if it's "inline"....it's flowing. It's currently (in the photo below) shown in the "bypass" position.
Originally Posted by jcassady
Here is the other side of the heater and the only other two hoses connected to it. They are connected to the tank.