This is copied from an earlier post I wrote:
Locate and check your water pump and see if has a valve and a loose piece of hose connected to it, them check the back of your water heater to see if it is equipped with a bypass valve. If so, it is a piece of cake.
Drain the entire system from the low point drains. Drain the water heater by removing the plug in the lower left corner and pull the pop off valve. You can also blow out the system with a blow out plug which requires an air compressor but this is not entirely necessary if you fill all the lines with the "pink stuff", RV antifreeze.
Once the system is drained and closed back up, you can close off the water heater with the bypass valves and suck the pink stuff up from the hose connected to the pump. Simply put the hose into the bottle, turn on the pump and it will pull the antifreeze through out the system. I put in enough so that each faucet, shower and toilet all flow pink. It takes only about 2 gallons. If you don't bypass the water heater, it will fill up taking up 6 gallons there alone.
The clear lines in the newer models make it easy to see if you are in fact bypassing the water heater and filling the lines.
After all the water outlets run pink, I put some in the drained black and gray holding tanks, and enough to fill the drain traps on each sink. I also put some in the toilet bowl, about an inch deep. This reportedly helps maintain the seal. Don't put any in the fresh water tank, just drain it to the extent possible.
This can sound complicated if you have never done this and it is hard to write a quick and easy how to, particularly when some one old and tired like me tries to!
If you have any neighbors with an RV ask them. Most are very willing to help out and lend a hand.
Ok fellow members, help out here in case I messed up!