While you can have 14.4 volts at the alternator output and probably 13.8 volts to the engine battery, chances are that very little amperage will go through the #14 or #12 wire going to the back bumper, then the #14 wire to the RV cord, and then to the RV battery.
Upgrading to a #8 wire with a 50 amp rated Anderson Connector, or a twistlock 4 wire plug (so that it does not accidentally get plugged into 120 volt power) you can get a lot more amperage back to your battery.
You would be much better off spending $800 or so to install a large solar system. You can buy a 140 watt solar panel for about $229. A PWM controller in the 20 - 30 amp range is only about $20 nowdays. Solar mounts can be made by cutting 6" long 2" angle aluminum and drilling 3 holes for #10 screws into the roof side and 5/16" hole for the solar panel frame side.
SunElec.comHave fun camping!
Fred.