I ran 2 12 volt deep cycle (27s) in parallel in my motor home. These were for running all 12 volt accessories plus TVs on inverters. It had a totally separate battery for cranking and running the engine.
I attended a technical seminar at an RV rally on house batteries and run life.
Turns out I bought 2 6 volt golf cart type batteries and ran in series for 12 volt house power.
To compare reserve power:
With the 2 12 volt ones,after running lights, TV and ceiling and exhaust fans all night, i had about 50% battery life the next morning.
With the 2 6 volt batteries
I had about 85% left the next morning. The batteries were a couple inches taller so I had to extend the tie downs but length and width were identical.
I don't remember the exact number but the technician. showed me the spec on reserve capacity on the 6 vs 12 volt and it was darn near double the reserve amps.
Bottom line the price was almost identical on the 6 vs 12 but the run time was about 3X for the 6 Volts.
Incidentally the seminar was hosted by Interstate Battery which is what I was running. The 6 volts were also Interstates.