Electrical

myredracer wrote:
A plug or receptacle will CSA/UL listed for operation at 125 volts (max.) and will be marked with a 125 volt rating on it. I would say that is what the OP is referring to. He probably just wants to plug into an ordinary 15 amp household receptacle and in that case adapters for RV use are readily available everywhere to go from 30 amps down to 15 amps. However...

You don't want to start plugging in any old extension cords that are laying around at home. Voltage drop needs to be considered or some serious damage can result. What is the voltage at the 15 amp receptacle you want to plug into, how far is the wiring run all the way back to the house panel and what is the wire gauge? An extension cord may need to be min. #10 gauge but depends on what would be running in the RV like say just the converter to keep the battery charged? Or running the AC unit and other loads in the summer? Plug blades and extension cords need to be kept in good condition and contact surfaces should be clean and shiny not dirty and pitted. Don't plug the RV in when the power is on.

The freezer in the shed will likely preclude running much more than the converter/charger.

A licenced electrician may very well not know about sizing an extension cord(s) for an RV and some other RV-specific things. It might help if the OP posts more info. and some here can help on adapters and cords.

OP wants to ADD a 120V AC outlet in his Rubber Maid Shed.
OP wants to run electrical wiring to Shed ----buried under ground.
OP does NOT want to use electrical power from his RV
OP wants a separate electrical circuit..apparently from a 30A circuit

OP doesn't want RV electrical adapters...extension cords

Per OP:
....getting electrical outlet to plug in my freezer without coming off of the camper....

..... I see there is electrical cable that that you can bury in the ground without conduit.