In a TM, a bypass valve is not needed, so you probably don't have one. Everything gravity-drains, so I don't know why the dealer put antifreeze in the system - lack of knowledge, I guess.
There is not any particularly handy way to back flush the water heater, as you can see here.
http://tmphotos.org/modules/xoopsgal...?g2_itemId=176
Depending on where the anitfreeze is in the system, you could rig up a hose to one of the hot water faucets, and run water backward through the heater. Or you could pull out the anode rod (it is accessible from outside the TM), squirt water directly into the tank, and let it dribble out the anode-rod opening. However, assuming that the system has pink (non-toxic) antifreeze in it, the easiest way may to gravity drain everything, then run water in and through the system, and just flush it out the normal way.
Bill