Most problems with RV's, new or used, are the result of defects or bad designs or poor parts suppliers (lowest cost)direct from the manufacturer. DIY modifications to fix these areas are to me a positive thing.
It is a false assumption not supported by experience that any RV comes perfect from the manufacturer and only needs to be serviced periodically and washed to perform with zero problems. When considering a used RV for purchase I research the known problems with the different brands and models and these have nothing to do with owner modifications done later.