It might take a bit of construction to make such a device. I would get one of these-
https://www.amazon.com/Electric-Mete...olt+watt+meter
One of these-
https://www.amazon.com/Watertight-En...JDRBY2RKB83SMJ
Two of these-
https://www.amazon.com/EKM-Metering-...R0YGHGAVS5C285
And you will need the appropriate wiring, plugs (male and female) that will allow this to be connected between the pedestal and your shore cord. The current transformers measure current in a ratio that will work with the recording device, one current transformer for each hotleg. I don't think any park can legally charge for power unless it has each pedestal individually metered. It would be better for them to just increase the rent to cover power usage. If you are familiar with wiring and electrical circuits, this should be easy to make, otherwise you might get someone who is capable to make a device like this.