Originally Posted by rmorrison9773
Going to add a second battery to my 5th wheel. I assume 12V Deep cycle size 24 and would connect the cables re to red and Black to Black. Correct
yes - red to red, black to black. That's "parallel" and will keep you at 12Vs.
Bear in mind the advice is to duplicate (or triplicate...) the exact battery you already have. Same size, same type, same AH, same everything. Optimally, they would even be the same age.
That's the *advice*.