In addition to what Larry said, it somewhat depends on how it was winterized.
I have always added a water heater bi-pass, water heater drain, and winterizing valve to all the RVs we've owned. With these in place it is a simple matter to winterize or de-winterize the RV. To winterize; simply drain the water heater and fresh water tank, adjust the bi-pass valves to run water from cold lines to the hot lines, turn the winterizing valve to pull out of the anti-freeze jug, with the pump on run anti-freeze through all the faucets and toilet flush valve, then with the faucets closed open the hot and cold low point drains and run the pump until the pink stuff comes out. Also, don't forget the outside shower if there is one.
To de-winterize with the WH drain, bi-pass and winterizing valve in place; fill the fresh water tank partially and turn the winterizing valve to pull from the tank, run the pump while opening each faucet until the flow is totally clear, then with the faucets closed run water through the low point drain until the flow there is clear, then close the drain valve on the WH, turn the bi-pass valves to their original position and fill it.
Without the bi-pass, the water heater may contain anti freeze which was probably fed into the system from the water tank. If that is the case, they will need to be drained and flushed until they are clear before going to the faucets and other lines through the low point drains. Without the water heater bi-pass valves, the hot water lines will have to be flushed with water that has gone through the water heater, and it will all come from the fresh water tank.
To the best of my recollection that is the way I've always done it. If I've forgotten anything or "recollected incorrectly", please feel free to correct.
Best of luck with it.
1994 30' Monaco Dynasty, 5.9 230 HP Cummins, MD 3060, 1992 Geo Tracker.
1996 Dodge Cummins 2500 with 1996 Lance 945 camper