Wow, late to the party as usual.
So, as I understand it, you want to make an extension cord that will be limited to "lights and battery charging" and nothing else, that is lighter than the 50 amp monster you usually carry.
If I have this right, you will be needing to wire up a custom cable with built in protection for over current.
I will need a photo of the camper's shore power socket (to determine if it is a female or male arrangement); the length of run you are talking about (to determine wire size) and a photo of the outlet you intend to plug it into.
I will do some research on how to do this safely to avoid overdrawing the power providing circuit and keeping the two male ends (if that is what it needs) from being hot without being plugged in. Most likely a water proof junction box somewhere close to the 20 amp male plug.
If you do not feel competent enough to make up a cable like this, your only option is to leave the 50 amp cable coiled up at the camper and a dogbone adapter to 110 male; followed by a heavy duty extension cord to the outlet.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW