Battle Born Batteries are one of the best choices on the market for lithium ion batteries and they became extremely popular in RV world. No doubts in my mind they have very high quality product and they are my number one choice for lithium ion batteries. In addition their website provides lots of valuable information I recommend anyone interested in this topic should read. They also have variety of system bundles that include most of the major components one would want to build a very robust power system.
Why lithium? It is a better battery, in my opinion.
AGM. I think it is still a viable option in some cases. If you have limited power needs, you don’t camp off grid very often, or you are just a weekend warrior single AGM might suit your needs. Most of the class C rental RV’s come with single lead acid battery mainly charged white driving and from shore power. But if you are building your own adventure vehicle you are probably lithium material, ready for more capable system.