Just curious why you came to the conclusion that you can't use the A/C relay?
I used a battery isolator from Hellroaring Technologies...hellroaring.com...in our F700 Toterhome and it worked very well.
Battery isolators have the issue of dropping voltage just enough that the house batteries won't charge well. Brian Elfert
I know I missed this part but have to ask?Brian,What voltage will your house batteries be? 12 or 24 volts?I am guessing that your bus is 24 volts. So the idea of a battery separator or isolator may needto be thought out a bit more.If your house system is 12 volts then there are things that must be considered and done differently.