I've been working on my ballast bag install on a SAN with tanks. I added two sacks in the rear and one in the ski locker.
I've done it 3 ways now so I feel like this way is the best one yet. In the rear I installed 2 ball cock valves on each side and 2 additional vent holes each side, but I kept the original fill/drain pumps.
I'll post a picture later but connect everything in this order. From the tanks connect the Over flow 3/4 Vent tube to a tee, then a ball cock, then the bag. On the tee connect a ball cock, check valve to thru-hull. Vent the sack with a seperate hose and thru-hull elbow.
With this setup you can either run the stock tanks, or any sack you want.
For the ski locker I put in a tee after both pumps and run a hose up front with a ball cock in the locker. I vented it out the cooler drain hole.