How long can a DP sit until problems Set-In?

If properly stored, both a diesel and a gasoline engine can sit for a long time. The type of engine makes zero difference, if they are properly stored.
If, they are just parked, and not prepared, with the newer fuels, both diesel and gasoline will start to break down after a few months.
To properly store an engine, the intake and exhaust needs to be sealed. The fuel needs to have a stabilizer mixed and circulated throughout the entire fuel system.
After that, you are now talking about things that applies to all coaches. For a coach to be road worthy(not a piece of construction equipment that is used off road) the tires will deteriorate, wiring can be rodent eaten, and various other aspects of the rig can and will go downhill. If stored outside, especially in a hot sun baked environment, any sealant or coating used on the roof could be close to failure. The AC units and generator could be infested with insects, and rodents could now inhabit all compartments within the structure.
I have dealt with vehicles that have sat for 40-50 years and the engine is usually the last of the problems, especially if it was somewhat prepared to be stored.

Depends!! Is it parked where rodents are going to move in? They'll eat insulation off of all the wiring causing electrical shorts and potential fires. They also strip the covering off of the heater ducting leaving the wire skeleton. I have observed on occasion where they have chewed thru plastic water lines. They pack in all sorts of junk like gum wrappers, plastic utensils and you name it for their nests. They leave their droppings all over. This enhances the threat of Hanta Virus. They will move into the engine air cleaner, the heater, the furnace, the water heater, and on and on. In short leaving one parked for long periods is not good.

Ours sits from November - April for the most part. Many take the batteries out. I leave ours in and every two weeks I start the generator for 1 hour ( with a load) to keep the batteries topped off, and run the MH engine for 30 -45 minutes at high idle each time. I will cycle through drive and reverse.

If there weren't snow and salt on the roads I would drive it. Many Diesel engine sit for longer than that. But I agree that more use is far better for the engine. We add a biocide and winter fuel conditioner to our fuel tanks for storage and keep the tanks full.

What your talking about is a problem every buyer Moho's must consider being gas or diesel. Low miles and age are not good. There is a break even point for age and miles that you have to establish for yourself. Also care of the vehicle. I bought my 99 last year, but the guy I bought it from was a maintainence nut. Keep records of everything, rhat is clue to buying used, how was it maintained before. If no idea, WALK AWAY.

Contrary to what most will post, a diesel engine can sit for years and crank right up with fresh batteries. This past month I cranked an excavator that has sat for five years on about two turns. Used it today to unload a heavy attachment that I'd had delivered. Back in the 70s I used diesel fuel that was 10 years old and I'm currently using diesel in my equipment that's five years old.
Now tires that have sat and not used regularly are not worth taking the chance that they might still be good. You can probably get $100 each from a farmer or logger for them as a blow out on a trailer is a inconvenience.

I've known coaches that have sat in one spot for 15 yrs and started right up...now I wouldn't drive it myself....just sayin...it depends...

IMHO, there's too many other things to worry about...:W..Dennis