While the 8D is a fantastic house battery, they are extremely heavy. I had one in a boat in the past and I doubt that I could lift one now with my bad back. They are standard lead-acid, but have a ton of reserve power. They also take a very long time to recharge, so a 'smart' charger electronic type needs to be availabe when you get to the dock.
AGM's also work exceedingly well, however I've never been a big proponent for them because of their cost. I'm just a cheapster when it comes to this stuff, and good quality lead-acids are expensive enough. Oddyssy's or Optima's are fantastic, with a nod to the former. You can pack a LOT of reserve AH with the compact oddyssy's.
A single G27 deep cycle is just fine for my little boat, but for the RV, when it's two house batteries gave up (very quickly, I might add) I went with two golf-cart batteries from Sam's/Costco. I can tell you they work absolutely fantastic. Lot's of folks go this way and it's a cheap and super duty way to get max power. They are taller than a standard battery, so you have to make sure you have room for them height wise. I'm not sure about battery boxes, I would assume they're available but have not made that conversion in a marine enviro. They are also well built to withstand the rigors of a golf cart, so they have real heavy plates, and they are deep-cyle rated so they'll stand severe cycling.
