What odd jobs needed on site?

Whether you would be able to carry out some of those jobs on a site is perhaps debatable.  Why not investigate becoming relief wardens on various sites?

I do not think,you would be able to do any sort of work you are looking at on club sites, for a start caravan washing is frowned upon on club sites,and any sort of maintanance has all sorts of H&S "problems" and you would also have to be covered by your own insurance and certificates of competancy,

You may be be able to "help"out on sites as a volunteer which can help with your pitch fees, contact HO and see what they say, 

Ps you would also need to be a member of either of the club's 

Other than that you would have to contact the owners of commercial sites 

To work as you suggest, TB, will involve carrying public liability insurance amongst other things. Caravan cleaning is not allowed on club sites and I’ve never found anyone who greases their chassis. Any necessary lubrication will be done if/when vans are serviced.

Sorry if I sound negative but I’m not sure you have fully thought this through and it’s better to discover pitfalls sooner rather than later. Good luck, anyway. 

TB I can only echo what has previously been written. Perhaps try commercial/private sites? Most caravanners tend to get an annual service done on their vans and unless you're a competent electric or gas fitter you are legally not allowed to go anywhere near someone's van, particularly on Club Sites.

There's many an occasion where I would have paid for someone to clean my van too, but site rules forbid.

I wish you good luck in your quest, but as they say on Dragons Den - 'I'm out'

The cleaning of caravan exteriors would find plenty of custom, but you would need to offer your services to people who lived near to whatever site you were staying at. No idea how you would get in touch, as people don't usually stay at a nearby site!