BTW, take a lot of care if you have to lower that battery to test it or swap it. It is very heavy and very awkward to safely lower it. An automotive floor jack is a decent help, but the battery can be unstable balancing on the head of the floor jack.
Herb
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SMB-less as of 02/04/2012. Our savings account is richer, but our adventures are poorer.
I've seen a few vans with a house battery on the side but most (done by SMB) are mounted under the van by the rear axle or inside as shown. If your starting battery is under the hood then yes the house battery could be mounted on the side rails. Make sure you measure the battery box before you purchase the new battery. The Ford battery boxes are too small for a standard 4D AGM battery. Herb is correct on a battery mounted in those side boxes and they can be slightly difficult to drop but looking at that picture it looks a lot larger than the Ford boxes.
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2006 Ford 6.0PSD EB-50/E-PH SMB 4X4 Rock Crawler Trailer