Once when I was at an Onan Dealer they were doing an oil change on a DP and he said it cost's $1000.00 just or an oil change!! I went into shock thinking my gas oil change doing it myself is less than $100.
That's pretty steep. I've not checked this year, but my last oil, filter, lube at my local Transwest was $250-300 (I think it was the 'A' service) and the other one ('B' service) was closer to $550, but it include many other things that need periodic service including fuel and air filters and some other odds and ends. And the bonus is that you only need to do the oil change about every 15,000 miles, not every 3,000 or 5,000, so at most I do it once a year.
Oil and filter on the generator (it, too, is diesel) is in the $200-$300 range too, every x hours (I forget if it's 50 or 100 hours) and gassers need this too.
The above is with the dealer doing the work. Doing it yourself will be a lot less and, while there are a few things you'll need to learn, you certainly can buy the 20-30 quarts of oil and the needed filters and do it yourself, saving a bundle, just as you would on a gasser.
Yes I am insisting on a new WHATEVER I get!
HA! I just have a thing about used. Just me.
I do understand that, but a gently used 3-5 year old model can last you a long time with minimal problems, and be MUCH more comfortable on the road, as well as much more resistant to crosswinds* than the gassers (design difference), much quieter up front (not silent, just a lot quieter) and is generally much better made than most gassers.
Also note Isaac's note above- it's not certain that you'll have problems as he describes, but it's a rather common experience.
* The long rear overhang on a gasser plus the fact that it sits much higher, and is generally lighter, means that crosswinds and passing semis affect it a lot more than most DPs. My first class A was a Bounder, gas, on the F-53 chassis, and one day I left Trinidad, CO northbound on I-25 headed for Denver and about 20-30 miles north of Trinidad I hit very, very strong crosswinds (frequent enough that they have warning signs) and actually had to slow to 30 mph or so, for fear of being blown over. We stopped at the Walsenburg motel for the night. That set me on the search for a DP.
Since getting a DP I've encountered winds just as strong (maybe stronger) in the same area, as well as several other places, and was able to maintain normal highway speed (65 mph) without much strain. There's that much difference when it's windy. It's not that you don't feel it in the DP, just that it's not a particular problem, compared to the gasser I had.