Camper outlet to power Shed
A feeder can't be a branch circuit.  

However, this would require AHJ approval, but I suppose you could run your underground line to the shed using normal &amp; proper wiring methods, and surface it right next to the RV receptacle.  Install an inlet there.  Then have a 3 foot long jumper cord that you plug in to the inlet and outlet to feed onward to the shed.  Don't even think of doing this until you've talked to the AHJ and gotten a general sense of whether they'll approve it. 

I would more expect the AHJ to ask "how much harder would it be to simply continue the line on to the panel and do it properly?" You'd want to have a pretty good answer for that. 

A more viable option would be to remove the RV receptacle altogether, extend off that junction box the feeder to the shed, converting the branch circuit to a feeder. Then from a subpanel in the shed, install the 50A RV receptacle. If this is wired with #6Cu or #4Al, once it's hardwired you can bump the breaker to 60A, or even 70A if it's THWN-in-conduit.