Phil.... you want to use TRUE deep cycle batteries as large as you can fit in the dimensions of your battery tray. In 12V batteries the sizes are Group 24, 27 and 31. Get the highest # that will fit.
Suggest Trojan brand IF you do a lot of boondocking. Suggest EastPenn Mfr. Deka or other house brand starting with model # DC .... if you mostly stay plugged in.
Stick with wet cells for the most bang for your buck unless you are prepared to spend at least twice as much for AGM's so you don't have to water them.
If watering is a chore...suggest you get a pro-fill type system to make it easy and convenient.
AGM's are a good choice IF you are setting up for boondocking with a BIG charger. They are also a good choice if you leave your coach unplugged and unattended for weeks or months at a time. They MUST be recharged to 100% or you will lose cycle life.
An alternative that I do not support but many do is to go for a pair of golfcart batteries (6V) hooked up in series to provide 12V. This provides a large and excellent deep cycle bank at a very attractive price and you can get such batteries from Trojan or from Sam's Club/Costco at good prices.
I like these in 4 battery banks...but worry about 1 bad battery wiping out the ability to camp with a 2x6V battery bank and 1 dead battery. Others are less conservative about it and remain happy campers.
Hope this helps.
2015 Georgetown 280DS
2014 Vespa LX150 2014 StellaAuto 125