I do a lot of caravan park testing. If it is a holiday site, well it's the wrong time of year to be turning off the power as you'll end up with a screaming mob after you because they can't watch Jeremy Kyle whilst on holiday with the kids!
Are you I & T'ing both buildings and the camping field?
Buildings are usually toilet blocks - so areas of higher risk. Then there is usually an office and sometimes a 'leisure' room/games room.
Camping field - you'll need to test the earth rod (and in my opinion that means a proper Ra test not a Ze). Chances are it will be 3 phase so you will need to Zs at the origins, RCD the time delayed main RCD unit(s). Zs's at DU's and then Zs and RCD at hook-ups. If they let you you should also be completing continuity and IR tests, but as I say if they won't let you isolate the power you will be unable to do this.
What do you mean by £5 a circuit? I presume you mean £5 a hook-up? But what are you charging for the DU's and the main boards? It's difficult to say without knowing the size of the site but the jobs going to take more than a day if you are on your own, but at £5 your not doing much better than their previous testers?
Don't assume that the previous companies test results are accurate nor accurately labelled/recorded. Is there a diagram of the electrical layout? Are you familiar with 3 phase and labelling/annotations conventions - L1/2 etc etc? It will probably be greater than 100A. Boards are often old, badly labelled (if at all) and do not necessarily have RCD protection for buildings. Then there's the issue of the historic labelling you have doesn't match what you see because the owners have renumbered all the pitches.
The site owners will only want the bear minimum to satisfy the LA conditions of trade and I doubt they will be happy to fend off an unhappy mass of campers without any electricity.
Be prepared to get ****-wet through, be prepared for your meter to die due to inclement weather and be prepared to get an ear-full of grief from campers whose son/grandson/nephew is a better electrician than you and earns zillions of pounds a year doing something less demeaning than testing out campsites.
I only do it cos I'm a masochist