When I had the dealer add the two 8D batteries it was because I thought I had seen in the manual that I got online that the Sky Deck comes with two 4D batteries, but your suggestion of the 6V batts is smart. I went looking for that reference in the 390 owners manual (that includes Sky Deck RVs) and it says on page H-1 that the RV came with the 6V batteries like you said, but on page H-42 (same chapter) it said it comes with 2 4D batteries.
Of courses I did notice several typos in the manual I have and there was the section on extended living in the RV where it called it a trailer, so obviously there is not a LOT of editing of this manual before it goes in your coach. I will probably sell the RV before the new 8Ds wear out, but I will do the Costco thing if I ever do need batts again. That's the kind I have in my current RV.
The advice on the fridge is smart too, although you could easily get a house fridge cheaper than a marine one either will use a compressor that is MUCH more efficient if you have a nice solar system. If you are relying on cheap LP, like I plan to do, the silent fridge is nicer than the generator running to top up the batts before "quiet time". My current RV could easily run on solar as its only got a 5.7cu.ft. fridge. The new one, the Sky Deck, is the big double door, so is that 10 or 12? Would need a lot of roof to mount all those panels and I currently plan to use the roof for parties.