Mad manx, one problem is we don't know what sort of RV lifestyle you are contemplating - full timer that is out on the road for weeks or months at a time or weekend warrior that might do one session of three weeks on vacation plus a couple of weekends every now and then. Then for each group there are full hookups every night, full hookups at least one night in three or four, and then there are those who spend weeks or months without getting anywhere near mains hookup. No single solution is correct for this whole range.
For one of these classifications the cheapest throw away discount starting battery will likely provide the most cost-effective solution. At the other end of the scale, the best deep cycle batteries plus lots of solar would be very expensive but actually prove to be the most cost-effective on a per-night basis. Then there are AGM and Gel and conventional and also LiPO4 ( or did I miss some letters or numbers) that are now available and are highly regarded by at least a small band of early adopters in Australia.
Don't forget that all normal vented flooded-cell batteries need to be sealed from the living space and properly vented to the outside and while some batteries have provision for vent tubes, most others do not so the whole battery compartment should be sealed and vented.
