Read the guides that Jim referred to, but in a (large) nutshell, here's my process. First I use my small air nailer compressor to blow the water from the lines, opening one faucet at a time inside the rig, as well as the outside shower (even though we never use it). Then I open the two low point drains and blow them out. The third one you mention is probably the fresh water tank drain and it will not be affected by this process since it is only a gravity drain of the tank. Draining it should also clear the line from the pump to the tank. Also remove the drain plug for the water heater, open the pressure relief valve and drain it. Be sure it's cold when you do or you may get burned. In more southern climates, that is sometimes enough.
For Michigan, however, air blasting is not quite enough, especially if you have a washer/dryer. So, I connect a supply of antifreeze (pour it into a 5-gallon bucket) and run a line to the winterizing intake for the water pump and pump it through the system, opening each faucet, one at a time, until a clear stream of AF comes out. Flush the toilet as you're pumping AF, too, to protect the plumbing and valve on it. Be sure to bypass the water heater or you will waste a lot of antifreeze filling it up. It is not necessary to put antifreeze into it, just leave the drain plug/anode rod loosely in the hole to keep any crawlies from getting in. If you have a washing machine, it is necessary to actually run it while pumping the AF so that the water distribution assembly, as well as the pump, get protected. I short-cutted this one time and paid the price. Take some AF when you're done and pour it into the toilet bowl to cover the ball valve. That will act like water to keep any smells from finding their way back up into the rig, as well as protect the ball valve. I also like to dump some additional AF into the drains to be sure the P-traps are protected.
The black tank flush line does not require AF. The line from the anti-siphon valve to the tank will drain by itself. The line from the UDC to that valve may contain water, however. If you still have the standard flow restrictor on the intake, it should drain by itself. If it's a check valve, press the little tab in the center and the water will drain out.
Do not pump or dump antifreeze into your fresh water tank unless you want to spend a lot of time trying to get it out.
In the spring, I again use the compressor to blow the AF out, collecting the output from the low point drains in a bucket and running the faucets. I then pump in a mild bleach solution through the pump to help clean it out. Once at our seasonal site, I let the city water run until no odor of chlorine is detected. I also sanitize the fresh water tank, once there, and leave it about 1/2 full. The reserve in the tank is in the event of a power failure at the CG, so we can still use the toilet off the pump and battery.
I did the draining/air blasting process earlier. Today, I'll be going back to the storage yard to pump the AF in before temperatures get too cold to work outside. When I do this type of maintenance, I have to bring my 2K genny along for power.
Others may have different procedures, but this is what works for me.
