Ive been running an ARB fridge just off the engine battery this summer, works ok as long as I was DD'ing the van but definitely needed some more battery power. My last van had an isolated Group 27 mounted on the passenger frame rail. This time I made a different tray that through bolts the frame. As of writing this all 3 batteries (underhood) are tied together without isolation. The ARB is my only draw and it has low voltage protection built in.
Plans are to run 2 Group 29 regular lead-acids in this tray, with charging from the alternator (through an isolator) and 200watts solar on the roof.
Pop Top Updates: Figured out my awning mounts and subframe to van attachment. Hopefully get the final subframe assembly together this week and get back to the Aluminum top