The $400 for the install I believe is to wire, install tie downs in the bed, and install the camper. Just to wire the truck with the 3 prong plug FWC uses is $75 plus some labor ($140 I think is what I paid) . The camper is a simple system and if you want to use your existing RV plug you just need to bring the male end with you and install it on the camper. You can google up the schematic for the wiring of the plug and just match it up. Not difficult at all.
I don't think it's a big deal but Stan said me they perfer the "FWC way" as they know it's 100% reliable. He eluded that some folks may have had issues with using the RV plug in the past... but did say others have had no problems. Since I was at the factory to pick up my used camper, I just had them do it their way so there wouldn't be any problems. I'm still not sure what problems there would be since that RV plug is on (should be) it's own circuit and already isolated from the vehicle battery. If I was in your shoes, I'd use the RV plug. my 2 cents....