FWIW, last year we visited Haven Rockley Park.. They said if we were able to plugin, charging from the van would be fine (at the time, we thought we had several charging options; but when we got there we found them all to be bad!). Luckily our car was parked right outside our bedroom window. We didn't really think about the possible issues with this and just plugged in and charged overnight almost every night (a Renault ZOE, using the official Reault granny lead). I didn't think to check whether the sockets got hot, but fortunately there were no fires!We had no option but to charge at our park, as we arrived with 0 miles remaining.
Checked with site staff and they said charging from the van would be fine (on this park - others may not be).
We can't unplug our blue lead and charge directly from there, as it would wipe the credit on our meter. So we charged using an RCD plug from a 3 pin in the van, without an extension cord, and with everything else unplugged except the fridge. The 3 pin socket and RCD was slightly warm after an hour, so we shut it off and let it cool before doing another hour. That was enough to get us to a public charger.
Definitely keep checking the socket if you try this (underneath the plug as well as around it). IMO it would be risky to charge overnight, but I may be extra cautious.
In future, we'll be nervous about booking without having confirmation we can park next to the van (unless we know there are reliable chargers nearby, and that counts out CYC and Polar!)