Who to believe?

Also, you would do yourself a huge favor and get two 6 volt deep cycle batteries and connect them together.
Believe it or not two 6 volt deep cycles will last four times longer than a single 12 volt deep cycle and twice as long as two 12's.
Found this out in our RV days when we went were there were no hookups.

A lot of people I knew back then had their RV back in the dealerships right after they took delivery for all kinds of reasons.
They also have wiring that sucks because they used wire with so much aluminum in it to save weight and expense.