It's easy for this topic to get over-complicated, but if you are plugging to a typical household outlet, you have a max of 15A (amps) available at the outlet, assuming nothing else in the house shares that circuit (unlikely). In that case an extension rated for 15 amps over a length of 25 ft should be sufficient when added to your RV shore cord. The 12 gauge cord that John recommends will do nicely and a heavy duty outdoor-use cord like that is available most places where heavier duty extensions are sold, e.g. Lowes, Home Depot, hardware stores, Sears, Harbor Freight, Menards, Amazon.com, etc. Walamrt may have them, but read the package carefully. The key description words are "12 gauge" and "outdoor".
If you need an adapter to plug in, Walmarts have them, 30A female to 15A male. Below I've linked to the puck type or the slightly longer dogbone type (I prefer the latter).
https://www.walmart.com/ip/Road-Power-30-15-Amp-RV-Power-Adapter/16817350
https://www.walmart.com/ip/Road-Power-RV-Cord-Adapter/16817356