Call a Mobile RV tech is you must. But typically, 2 house batts are generally 6 volt deep cycle. So find a big cable that attaches to the metal of the chassis. Attach that to the NEG- of one of the batts. Then find the short jumper that's around there somewhere to attach that batts POS+ to the NEG- of the next batt. Once that's done, than use DMM (Digital Multi-Meter) available at most big box stores for less than $10 USD, to measure the voltage from the NEG- to the first POS+. That should be near 6 volts. If it's not, let us know what it is before proceeding. Then measure from the NEG- to the 2nd POS+ of the second battery. That should show around 12 volts.
Now find a big RED cable and connect it to the empty POS+ terminal.