Make sure you are comparing apples to apples.
When considering the decision between 6 volt and 12 volt banks, these items should be considered:
1) Weight - Good quality TRUE deep cycle
batteries are heavy.
NOTE - most batteries delivered with travel trailers are not deep cycle batteries; they are the so called "Dual Purpose Marine" batteries. Marine batteries have thinner plates than deep storage batteries because they are expected to be able to START the boat engine and then provide some limited storage for accessories at anchorage. BUT THEY ARE LIGHTER and CHEAPER so they get put in by the dealerships (not FR who ships units without batteries).
NEVER use "automobile" batteries. They are for starting only and have almost NO storage capability. With the engine in ACC, turn on the radio and play the radio for a few hours. You won't have enough oomph to start the car.
2) Cost - True Deep Cycle batteries have more than twice the lead of Dual Purpose batteries and three times the lead of starting batteries to hold that capacity. Lead is expensive.
3) Amp Hours - Whether provided by 6 volt or 12 volt batteries, AH is EVERYTHING. Nothing else matters to determine how long a battery bank will last when being used.
NOTE: CCA is Cold Cranking Amps and that means you DON'T want that battery. Dual Purpose batteries have CCA and RC numbers. RC stands for "Reserve Capacity" and is in MINUTES of life when disconnected from a charging source. To Convert RC to AH you must multiply the RC by 0.4167
An RC of 200 minutes is equal to a storage battery of 83.3 AH for example.
4) Battery size - There are several "common" sizes (24, 27, etc) each battery size increase has more plates, more weight, and is bigger physically that the previous size.
If you upgraded your stock 12 volt batteries to Trojan 12 volt deep discharge batteries of similar size you will be VERY pleased with the result. Similar sized Deep Cycle storage batteries have about 50% more capacity than Dual Purpose batteries.
Buying large batteries that won't fit your camper is wasted money.
Measure many times before you order.
5) Venting - Make sure your vents are large enough to handle the gasses created during charging larger banks. Battery gasses are flammable (explosively so in confined spaces) and have been known to set off your propane detector.
I am sure there is more, but need to get going...