The test we ran with one of the batteries out, was with the generator on and 2nd test (with Gen. off) plugged into shore power. Jacks would not work normally without BOTH batteries installed. Which led me to believe that the jacks use battery power only/always. I don't think the jacks are used through the inverter (as mentioned above to much amperage). I do know some of you with smaller campers use the smaller Atwood electric jacks. Mine are the big boys. If you can tell me why having one of the batteries out of system would stop shore power from running the jacks, please do tell. Our toilet and shower are directly over black tank, the grey is up against the rear of camper, both in the basement of course. Hope this adds some clarification.