poor connections between batteries will inhibit current flow
go to harbor freight and buy their 2.99 digital voltmeter, pull the battery to battery cable and, with the v.m. in the 10 AMP range (READ THE INSTRUCTIONS), place one lead in on the cable and one on the battery terminal it came from. if you have a low current indication (=< ~ .2 amps) disconnect and set up the v.m. for low current measurement to see what your actual draw is.
I'm guessing if you're killing a new battery overnight the draw will be significant
regardless of what you have for a drain h.f. also sells battery disconnects, won't fix the Problem but will save batteries