I have a 2001 horizon so a bit older but the same platform for the most part. On the valve adjustment, I bough my rig in 2015 with 33,000 miles on it and assumed it was not done. I am handy with tools and there is a great video on line from adept ape that shows you how do do this yourself. If you have ever done a valve adjustment on any motor this process is the same, this is not difficult to do if your handy. Long story short, I did this myself and my valves were not out of adjustment. So if its not running poorly i would let that ride for awhile, Otherwise its about 300.00 or more to have it done.
One the ECM, I would just call a dealer on that get the 411 on the necessity and cost.
Lastly on draining the the tanks i have done this several time and I have never had moisture escape. Having said that i have been on top of my Air dryer maintenance and filter replacement. They are supposed to be replaced every three years. so I would start there. its also easy to do yourself. You can order the parts on line, use a strap filter wrench and swap it out. also lots of videos and DIY on line.
2001 Itasca Horizon
24' Enclosed Car Hauler