The reason for staying with the 6V golf cart batteries is they have no CCA, (cold crank amps) since they are not starter batteries and more CAH, Capacity Amp Hours, which is what you need to power your 'stuff' while stopped.
Since it sounds like your uncle didn't do any oil changes himself, I would take your coach to a Speedco and let them change the oil and filters, fuel and oil and let them teach you where everything is. You'll need the experience of changing a fuel filter anyway. It will come in handy if you're stuck on the side of the road after getting some bad fuel. Speedco will let you in the pit and show you what they're doing. You can mark your dipstick after they fill. They'll also check your coolant SCA while you're there. Plan on about $300 bucks.....Dennis