There's not very many DPs that are 36' or less. Most are much longer than that. If you need one that size and have a budget around $100k like you said.....I can see why you're having a tough time finding many DP options that aren't several years old.
Yes, there are indeed a lot of very nice (new) gasser units available to choose from for that $$ or quite a bit less. When it comes to RVs, I too prefer new, so thats what I'd be looking more at, if I was in your situation.
As to things a DP has (besides more pulling power), that you don't get in a gasser:
1. Full air brakes, and air suspension
2. Somewhat easier leveling, since you can dump air and drop entire coach down some before you start leveling
3. Much better ride when on the road, due to afore mentioned air suspension, the chassis its built on, and engine being in the back making it much quieter.
..However, as I'm sure you've seen, these 'pros' come at a very, VERY high price, compared to similar gasser models. I'd love to have a rear engine unit with full air suspension and air brakes. However, like many, I don't want those things bad enough to part with nearly $100k MORE up front for the initial purchase, and several thousand more over the years in extra maintenance costs the DP will incur.