If you go to walmart and get a 10+ amp portable automatic battery charger it will get you through the trip just fine. Just slap it direct on the battery terminals. Pick up an extension cord if needed.
Otherwise post the brand and model of the converter or inverter that should be supplying 12v to the RV. Basically it is 120vac in and ~13.6vdc out. bench test it but I think you will be buying a new one.