This turned out to be a great success. I ended up buying an AIRHEAD AHP-12HP High Pressure Air Pump (12 Volt) and using the pressure relief valve to limit pressure to 1.4 psi. The procedure I used was to run the system dry using the water pump, then disconnect the water pump and pressurize the lines with air from the outlet to the rest of the system, all faucets open. I removed the level gauge sender and suctioned out the residual water in the tank using a shop vac tip extension that had been thoroughly washed and sanitized.

I still used pink antifreeze in the shower sump and heads, and the -100 stuff for the engine raw water.