Hey RoyB-MA... Sounds good... It certainly is the way to go for my much smaller 12-foot box OFF-ROAD setup... I don't have that many water lines to worry about in my setup and the beauty is I go back to using water again by just adding water to the fresh water tank the next day...
I carry my tankless 12V air compressor in my truck under the rear seat area and setup on the tailgate of my truck with the compressor where I can clip into the batteries mounted on the trailer tongue area. My long air hose fits the city water inlet where I use one of those air adapters. Using the tankless compressor allows me plenty of time to go open up water spigots one at a time and not build up any high pressure in the lines... The one only P-trap gets a douse of pink stuff.
This is a ten minute job for me to do - The longest time used is draining the fresh water tank if it was full to start with. This also drains most of the hot water heater contens using the low water drain point if I open the Hot Water spigot.
I just read another post where the guy found more water in his low drain point after he finished. I too do what he and others do and check for more water after I have finished... Sometimes I find some as well... I also do the manual opening of the Hot water relief value to allow it to drain better through the low water point for the other lines. I don't worry so much about leaving some water in the hot water heater. It has lots of room in it for expansion from a freeze.
Nice install of your compressor...
RoyB-VA
Roy Ken