What you're contemplating is not a subpanel. It does not need a separate ground rod. Just use 8/2 UF with ground and you will be fine. You can get a lockable, water-resistant outlet box with the appropriate receptacle at most RV dealers. That's exactly what I have.My father-in-law said i need to run a ground wire. One soild cooper wire 50' long. Seems excessive for a trailer, any thoughts. We can'nt use cooper rods out here, after about 6" you hit soild rock.
Thanks for the information