I have an outdoor electrical outlet that powered my 5th wheel RV trailer. 50 amp style plug. It was installed by a licensed electrical I sold my trailer and want to power a shed. The shed will have a small 25 gallon hot water tank and probably 110 air conditioner along with outlets for tools. Can I run a power cord from a small breaker b and plug int ox
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 & 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.
Can you? or Should you? You could, but it probably wouldn't code compliant or safe. If you have 4 wire service (2 hots, neutral & Ground) to the existing outlet I would suggest installing a small sub-panel in the shed and hardwiring it. You'll need to leave the junction box and connections permanently accessable and drive ground rods for the sub-panel, but that would give you the most flexibility in the future.