How to stop draining batteries when parked?

[14T]

I have a 2003 DS NQ0 399 with factory lights, turn signal, wipers etc. If I don't plug it in for 2-3 days the batteries will drain down by about 30%. I know all the lights will work with the key off when in in either the tow or run position. Even the back-up light will stay on if parked in reverse.
There is a 14 gauge wire coming off the positive terminal of the second to the last battery. (16V source) before the negative cable. I assume this powers the accessories and lights through an inverter.
I do know that the batteries will hold a charge if I disconnect the battery cable. What changes can I make to make sure Off is Off without disconnecting the negative cable on the battery? I try to keep it on the charger, but I don't always have that option.

[Sergio]

Did You disconnect the main cable or the accessories wire to test the parasitic draw?

[14T]

After flipping the switch to tow, I disconnected the ground cable from the batterys. I didn't disconnect the wire going to what I assume in an inverter. It is still connected to the battery that would supply it with 16v. Rain arrived and I didn't want to dissolve.