Mechanically the weak point is now the engine, with decades of using up the 'runners' when one goes bang, overcoming engine failure is now dificult. Without an engine all you have is a nice shiny trailer.
I am sorry but from personal experience over many years I cannot agree. RT engines seem to go on for ever, they were built to do massive mileage between rebuilds and in preserved use they will not experience the mileage nor, hopefully, the harsh treatment by employed drivers that didn't have to worry about the repairs. Use anti-freeze all year round to reduce internal corrosion and feed them well with oil and water and they should go on, and on, and on.
Engine issues that may arise are fuel blockages with contaminated fuel, and worn fuel pumps - both are easily dealt with.