Bang for the buck, you can NOT beat TWO 6V golf cart batteries. If you don't have a Sam's Club or Costco membership, get one, find a friend who has one, or simply ask at the service desk if you can have a "one day, free trial" membership !
Two 6V golf carts batteries should be under $200. Two marine/RV would definitely cost more, maybe even >$250 and probably has abiut the same or LESS storage capacity.
Ditto on that. People drone on about how Lithium batteries last 10 or 15 years but you can buy multiple sets of golf cart batteries for the price of a lithium bank. And golf cart batteries will last 1000 cycles if properly cared for. I interpret one cycle as one day. How long will it take you to boondock for 1000 days? I don't run my batteries down that low in the first place.