Al, actually use 2 gals. of A/F with equal amount of water per engine. Could interpret that as 4 gals/engine but that w/b a 50/50 mixture. Same holds true for genny. One gallon of A/F with one gallon of water.......2 gallons of 50/50 blend.
Re fresh water system; blow most of system out with compressed air. Maximum 3 gallons of pink stuff for fresh water system. Nothing for air conditioner; just compressed air.
This has worked well since buying boat new in '93.