I would say the sweet spot on used higher end DPs is 2006-07 with a Detroit or Cummins engine although I've saw a new 09 Monaco with a 06 engine at a rally... Monaco bought up a bunch of 06 engines for future use. Around 06 the TVs are digital although replacing a older one is fairly easy with a
LED TV and not very expensive. A 06+ unit should have fairly new tires depending on when the original tires were replaced.
In my 11 years of ownership I've spent the most on my 03 ISL engine because a 45 degree elbow on the intake was not put on properly, eventually letting sand get into the engine. Fortunately a intake valve adjustment loosened up putting me in a shop before it needed a major overhaul but still a $9,000 repair. Recently I spent $1,000 replacing a CAPS fuel lift pump which are know to start leaking. Turns out the lift pump did not need
replacing. I'm probably 1 in 100 who's had to spend serious money on their engine.
The next most expensive item has been my NoCold 1200 refer. Between a auto fire system, new Amish cooling unit, new control board, new ignitor... $1800+ in the last five years. I just finished replacing it with a home refer and here's my
DIY post.I'm going to buy a unused $700 old style Dometic AC/heat pump to have as a spare for the three that I have and are working well,
BUT the new style Duo Therm will not work with my control system. There are reports of $4000+ to change two units over to the new system
here. Not sure how much newer you would have to go to have the newer style Dometic AC but it could be as much as 2010.
Despite all of the above I see no reason that I will have these kinds of costs in the future as I plan on using this MH for another 10-15 years.