Camco 30M/30 Amp RV Extension Cord, cCSAus - Walmart.com
My wal-mart actually had a cheaper version in stock, it did not have the fancy yellow grip connectors, but was clear and lighted (bulbs burned out pretty quick) for around 20-25 bucks. That is not much more than a decent heavy duty extension cord.
I dont think anyone mentioned before, but basically all pop-ups came with 30 amp service once they started pre-wiring all the roofs for A/C so it could be added by dealer.
You already have the 20-30 amp adapter from prior owner, so I would just pick up the cheapest 30 amp cord as the few bucks more you spend over a regular extension cord may come in handy down the road. you will be able to leave the 20 amp outlet free on your campground pedestal for running stuff outside should you want to, and will have more power inside if you ever upgrade.
2013 Coachmen Freedom Express 320BHDS pulled by a 2005 F250 King Ranch CC
A rainy day camping is better than a sunny day at work.