Blimey Alan .. so many variables !I noticed this is your first post and you have been a member for just a few weeks.
I apologise in advance if you have been searching for some time but the best advice I can give you is make certain you have researched RVs before you even consider finding an engineer to inspect; and then research again. Ask questions on the forum. Dealers are known, history of many RVs are known. For obvious reasons you may sometimes get an answer via pm. Some forum members have RVs for sale. Your choice of RV may well influence the engineer you require.
There are so many variables: new or old, petrol or diesel, engine at the front or back, price point, length, full timing or holiday use, locker space, eBay or private sale with no warranty, dealer with warranty, layout, number of slide outs, weight and licence requirements, payload, Tyre age as new tyres can set you back £2K. One AC unit or two, are they heat pumps as well. Petrol, gas or diesel genny. Air brakes, non air or air over brake fluid. Air suspension, non air suspension or airbag assisted.Full conversion to 240v or still on 110v. Inverter and size of inverter. Washing/drier. Are the lockers heated. Etc etc etc.
The old adage of buy your third RV is very true, although I did meet a guy who on waking up with a hell of a hangover discovered he had, whilst drunk, bid on an RV and was the highest bidder.
All the best with your purchase. We all have varying experiences so just ask.