For house batteries it depends a good bit on how you intend to use the motorhome. If you need something more than a token battery, often the most cost effective in terms of capacity and longevity for the money are pairs of 6V golf cart batteries. It pays to shop around for them a bit. Sam's Club and Costco in many places have them for around $90 apiece, but sometimes local golf cart shops may have better quality or higher capacity batteries for about the same price...and sometimes the golf cart shops are much more expensive.
If your time camping will be generally spent hooked up to shore power or running the generator or on the road, you don't need much of a house battery. A single marine/RV battery is probably the least expensive. All it needs to do is keep things running while you run into a restaurant or hook up at the campground.
