The 30 amp site I strayed at had 15/20A outlets as well. You could plug into 30 with an adapter because it’s still only 110/120v but for a 50A you’d have to make sure your adapter only uses only of the hot leads to avoid putting 220/240 into your 20A system.

If you have an adapter from the 20A to plug into a 30 or 50A outlet. But not sure about the question. Are you saying the hook up you have feeding your camper looks like a reg wall plug? If so Amazon sells 15a female end by a 30A male end that would work.