I plan on buying my rv this year or latest next year, depends on my situation. I plan to buy a used rig and part of the reason I have been through my training is to be able to understand what I'm looking at. What I don't understand, like the running gear, I plan to have a diesel mechanic and maybe the qualified chassis maintenance facility look at it as well.
Just to set a reference point:
So, I will probably be looking for something like some combination of Newmar, Dutch Star, tax axle, comfort drive, hydronic heating, 2008 or younger. I plan to pay cash and probably around 150k plus or minus. That probably won't qualify as gently used, but it is out about 6 years or 7 years. I would go for older, but the comfort drive is a must have on my list and wasn't introduced until 2008.
I think it is primarily about maintenance and whether or not it was full-time quality rig when originally manufactured. Many say it is about the inside layout, but I think some changes can be made to the older rigs to improve the layout.
Seriously, the run into the ground concept makes little sense to me unless I'm looking at something that has been neglected and ill maintained. Say, heavily rusted out, horrible roof leak and delaminated/rotted out side walls, a blown engine, or at a junk yard. Something like that might be what I think as of run into the ground, in general. But so much can be done with well maintained, upgraded, rebuilt efforts ...especially a diesel, that I find the focus is better to be put on the details of the rigs condition at any point in time.
Taking one small detail, as an example, assuming a Suburban water heater, I would ask the owner some open questions, like do they use an anode rod, when was the last time it was changed, how many have they used over the years, and how often do then drain out the bottom 2 quarts of the tank. Right there is upwards of $1,000 to $1,200 or more to replace the water heater if the specific maintenance and the use of anode rods has not been done for that water heater, even if that water heater is working "today." That is why I can't understand general comments of a rig being in excellent or good condition...Seems to me you have to get very specific...at least that is the way I have been trained as an RV inspector. I'm leaving for a weak of advanced training tomorrow. Again another reference point.
I respect the experience of owners, but I don't think one can assume that means an owner has the specific expertise to know or desire to do all the specific maintenance needed on these rvs, nor the buyer to do the specific due diligence to know what they are looking at beyond, falling in love with their dream possibilities.
I have yet to read a specific limit as to how long rigs can last. A poorly maintained rig or a poor quality rig is always a potential breakdown in progress...and perpetually "run down"