The positive post is the one next to the "+" mark on the battery.
The other post is the negative post.
Assuming you have 2 batteries, if they are 12 Volts the positive posts will be wired directly together and the negative posts will be wired directly together.
If they are 6 volts you will see one positive post on one battery wired directly to the negative post on the other battery. The remaining positive and negative posts supply the 12 volts DC to the coach.
Ted & Melinda
2016 ACE 27.1
2016 Chevy Sonic Toad
2020 Chevy Colorado Z71 Trail Runner Alternate Toad