I have no idea what the load output would be useful for in an RV since everything is tied into the 12 volt battery system. It may be useful for other applications such as a fan in an enclosed area or whatever. In an RV you are all set up already for everything so adding solar is just to charge the batteries. Maybe someone will chime in with a use for the load output.
I have attached a picture from my controller manual that mentions the load output.
When I upgraded my unit to 400 watts I did a test. I fabricated a load tester where I could switch in increasing loads to see what my output was. My first test was just to connect the panel array directly into my load tester and measure the available current while watching the voltage. I then hooked it into the load terminals and tried increasing loads.
The load current pretty much matched what the panels were capable of. As far as battery charging is concerned, it appears that I can get close to the max amount available for load output. If I had something connected to the load terminals that would lower my available charge for my batteries.
Hope it makes sense. I recommend you download the manual for the controller you will be using. Good info there.
I am an electronic nerd and love this stuff. Gauges and gadgets, happy days!
Retired electronic technician (45 years in the field)
2017 Greyhawk 29W
solar & many other mods
wife (maybe I should have given her top billing)