What was the nature of the breakdowns? Think he needs to talk to whoever he bought it from, was it a dealer?

The dealer may have sold defective vehicle and some sales of goods act type rights may apply if bought from a dealer.

However there was an independent inspection which gave it a clean bill of health. The inspection appears deficient - so should they take responsibility for failure to spot the defects.

A claim rejecting the vehicle may work - but I can see very clear reasons that any settlement, if forthcoming at all, will be challenging.

GOOD LUCK - clearly not a happy situation!

Perhaps the pre-purchase inspection focused more on the condition of the camper side of things (internals and their electrics, water etc.) and bodywork rust and the common underside rot rather than the engine mechanicals?

The OP has not told us what any of the "12 breakdowns, each requiring major mechanical work" actually were.

This information would enable us to give more relevant advice. (Might also give an idea of the competence of the garages that carried out the "major mechanical work" which in each instance led to yet another breakdown).