Back in the day when all we had was top post batteries that just had the two large terminals, reverse polarity wasn't an issue. All positive terminals were larger than the negative so easy differentiation was possible. With the use of the threaded post terminals, reverse hookup becomes easier but every battery is still marked on the top with a (+) and a (-). The (-) terminal will always be attached to the RV frame and the (+) cable will be connected either through a junction box close to the batteries or lead directly to the distribution center and converter.
A hand-held meter set to DC Voltage will make the job of identifying which terminal is which and using the Continuity or Ohm Resistance setting will disclose which wire is ground (-) to frame.