Thanks everybody!
I ended up buying the batteries at Walmart - a pair of 27s. I was able to do it in one trip... marked all the leads as I removed them... and was surprised to find that tightening the tie-down straps was the hardest part... the clips are hard to juggle into their grooves while maneuvering the battery... and then the buckle of the strap is in a narrow space and hard to get fingers on... not hard, just lots of putzing around, complicated by it being 100 degrees and my oxygen concentrator overheating.
But thanks - the 27s fit just fine and everything is operational. Now my next project is to bring the rig to my home and work on that ridiculous issue of cutting a hole in my dashboard to get at the engine battery!
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