New DP Owner. Maintenance Budget Question

It depends on who you have do the service. It can be as high as $2,000 or as low as $1,000 for a full service including LOF (Lube, Oil and Filters - DP coaches have fuel filters, oil filters, transmission oil filter(s), fuel water block filters, filters for the air system). You will not need to do everything every year just keep up with the manufacture's recommended service schedule and use recommended fluids. If you are handy and have the correct tools it can be very inexpensive. DP coaches will essentially last forever if you take care of them. Also keep up on the other service related items such as resealing the roof every 6 months to at least annually. This means get up on the roof and look for voids or cracks in the sealant on the roof and add the recommended sealant (I'm a big fan of EternaBond). By all means keep the roof clean it will be vastly easier to spot any potential problems if the roof is clean.
Don't forget to check the condition of your tires especially the inner dually rear tires and don't forget about tire rotation if recommended by your tire manufacturer. Enjoy your coach!