Note: This procedure assumes you have a Suburban Water Heater. In Part 1, we drained the water heater and the fresh water tank.
Winterizing is much easier if you have a water heater bypass. If you don't have one, we highly recommend adding one. It not only simplifies your life, it saves you the cost of an extra 6 gallons of antifreeze necessary to fill the water heater.
You can buy a bypass kit at Camping World (or other RV store) to install yourself or you can have a RV shop do it for you. If you bought a used camper, it is very likely a previous owner or the original dealer installed one. Find your water heater inside your Roadtrek (since you just drained and flushed it in Part 1, you know where inside to look for the rest of it). Ours has a handy door in front of it, but it is possible yours may require removing a few screws in a panel to reach it.
A bypass kit is simple. It is an auxiliary pipe connecting the input and output sides of the water heater with valves that direct the water to flow through this pipe, bypassing the tank. Change the position of the valves as part of the winterizing process. The valves let you leave the water heater tank empty and still be able to flow antifreeze through the rest of the plumbing.
It doesn't matter if you start at the top or bottom, but turn each valve so the water will flow into/out of the bypass line and not the tank. It is possible to have from 1 to 3 valves in a bypass kit, so check it carefully.
Now turn the other valve. The handle is in line with the direction the water can flow. In this case it is parallel to the bypass line. That's it. You have now bypassed the water heater and will not need to fill it with antifreeze for the winter.
You may worry about a little water left in the tank after draining and flushing. Any water left has plenty of room to expand if it freezes, so it is not a problem. You are now ready for Part 3.