It is those hidden things that are most important to find, and a detailed inspection is helpful here. Also many / most first time buyers don't have a frame of reference as to how certain things should look, ie should that sealant be cracked, or is that bulge a sign of delamination in the side wall. In other words a newby might think they are buying a coach that is in great condition, but an RV inspector can walk up to it and see delaminating side walls, a failing roof, wood rot, and unsafe leaking propane system with expired CO and LPG detectors..
As to inspections being worth it on older RV's it depends, it sounds like this dealer is selling as is and is not going to fix any problems that are discovered, at which point the question is should you get an inspection where your only recourse is to walk away from the deal if major issues are found. If the drive train is your major concern you should have a mechanic familiar with this brand of chassis inspect the engine and related systems. In an ideal world you should do both, as a mechanic will likely have no idea how to inspect the roof of an RV, and it is too much to ask that RV inspectors know the workings of every brand of motorhome chassis.