Confused about Batteries

[Jack & Donna Smith]

I need a course in Batteries for our 1973 Coach. When we bought it
they claimed the batteries had just been replaced. It has 2 units in
the drivers side front compartment, one more under the entry steps,
and the 4th in the Generator compartment.
The batteries in question are all Walmart 12 volt Deep-cycle Marine
units of 675 & 875 amps each. (The big yellow ones).
Now 2 or 3 are bad or going south.
Most I have seen are 6 volt units. Why are these considered better
than the 12 volt ones.
Since I will be replacing these soon, which should I use, and why?
Jack Smith
1973FC31
SoCal