The real issue is cost and availability of TRUE deep discharge 12V batteries. "Dual purpose"/"marine" batteries are a compromise. Any battery that advertises Cold Cranking Amps (CCA) or Marine Cranking Amps (MCA) is NOT a true deep discharge battery. These will have a short life when used in an RV.
The only readily available and reasonably priced deep discharge batteries are 6V golf cart batteries (industry size GC2). You can probably read 100s of posts over the past year where others have said two 6V golf cart are your "best bang for the buck".
FYI - 12V golf cart batteries do exist (size GC12), but are NOT readily available and are $$$ !
