The 6V golf cart batteries are the way to go as they will have more capacity than a single 12V. The problem is do you have enough room to add more batteries? Do you have an inverter/converter to run her oxygen concentrator when not on generator or shore power?
You may already know this but.
Converter
A converter is an electrical device for converting 120-volt AC power into 12-volt DC power. Most RVs with electrical hookups will have a converter, since many of the lights and some other accessories run on 12-volts DC. A converter may also function as a charger for the coach batteries.
Inverter
Inverters are electrical devices for converting 12-volt DC power into 120-volt AC power. There are two basic types available: modified square-wave and pure sine wave with the former being the most common and least costly. They are available in sizes ranging from as low as 30 watts up to 3500 watts or more.
Bill