You're right about the drain valve, (there is also a drain valve on the WH), but I added pink antifreeze to all three tanks, after blowing all the lines clear and ran the pump, (don't forget the macerator pump), to make sure water would not damage the pump. I couldn't see a valve between the 2 water connections. I also used a pressure regulator to make sure I didn't exceed 60 PSI. Here is the Airstream Guide:
1. Open, drain, and flush the fresh, black, gray tank valves. Make sure all tanks are completely empty.
2. Run water pump a few minutes with faucets open to remove water from the pump lines.
3. Remove any water filter canisters.
4. Appling air to city water hook-up, keeping the pressure at 60 PSI.
A. Screw on the compressed air adaptor and hook airline onto the city water inlet.
B. Open the following valves/faucets, one at a time in the following order:
Note : Make sure to drain the toilet last on the inside of trailer as it is the lowest interior point.
Washer/dryer hookup (if present)
Note : Some older washer/dryer hookups have
a screen filter that must be drained.
Note : Hold the spray head open until air
Ice Maker (if present).
Note : You must unhook line to let the water drain.
All outside faucets
Note : Push foot petal down to open valve. If toilet is
equipped with spray head, hold it open until air sprays
Note : Depending on your unit, you may have a washing machine, icemaker, or dishwasher. The lines to these
appliances must be drained and blown out as well. If you have a tank flush, you must blow it out also. Refer to the
owner’s manual for more information on these appliances.
C. Disconnect the air line
D. Open the pressure relief valve then remove the hot water drain plug. Allow all the water to drain out.
E. Reconnect the airline to blow out any remaining water in the hot water tank reinstall plug.
F.Open all low-point drain lines to blow any remaining water out from the interior water system..
G. Remove compressed air and compressed air adapter.
If you are confident you have successfully blown out all the water from your system you can now add antifreeze to all traps
and the stool. One cup in each to fill p-trap.