Good idea - get in with the site owner - offer the service and go up twice a year for a long weekend - all travel and accommodation to be put in as a taxable expense!
I clean my non-static caravan with wfp and reckon you could do nicely out of it as an add-on.
Or if it's just windows go every quarter.
Being nosey, whereabouts in Wales and what's the deal? Site fees etc.?