I build off grid solar systems for a living. I have a few customers who have tried these types of things. They must not have been satisfactory or they wouldn't have called me to put in a full scale permanently installed system for their cabin.
Your right, it won't run an air conditioner for more than an hour or so. I would focus on improving the system build into your RV. Get a larger battery bank, some solar panels, charge controller, inverter, etc. It will cost less than the portable unit, be more convenient, and probably be more effective if designed and installed right.