My list of "to do's" could actually be quite lengthy, but I shall try to condense!
Obviously the regular automotive chassis items need to be checked, tires, fluids, etc. Newer unit, you should be pretty good.
You mention de-winterizing the pumbing, but you need to also sanitize and inspect the entire water system. Pressurize it with your pump and make sure it holds pressure without frequent pump cycling. Frequent cycling could indicate a small (hidden) leak. Put some (clean) water in both the black and gray tanks and remove the outlet cap to make sure the dump valves are not leaking through. Better to determine they are holding with clean water than not! Make sure you have all the items necessary to connect to the camp sewer. Lots of options there to consider. One thing I have heard is that if your unit still has the Jayco-supplied sewer hose you probably will want to upgrade.
Also make sure you have everything you need to hook up at camp electrically including a 30 to 20/15-amp adapter, and I highly recommend a surge protector. You might also consider a 30-amp extension cord. Some sites you may need one. For the generator I would suggest digging out its manual and following those instructions. More info there than I could relate here I'm certain.
Make sure you have a good potable water hose (or two). I also recommend a quality pressure regulator to protect your plumbing. I also carry a separate hose to use for my blank tank flusher, it is gray in color so I never mix it up with my drinking water hoses. You may want to obtain a spare filter element for your water filter.
Check operation of hot water heater, I believe a '15 GH has the tankless Girard. Make sure you are acquainted with the proper operation of a tankless heater since they can be quite "finicky".
Make sure refrigerator cools on both gas and electric modes. Will take some time to verify both modes. Proper flame (in the gas mode) should be visually verified.
Make sure all stove burners light and burn with a (mostly) blue flame.
Make sure your coach and chassis batteries are all in good shape, a load test would be ideal. If the coach sat in freezing weather with dead batteries they could have also frozen which will greatly reduce their life.
If your unit has levelers make sure you inspect and test the system. If yours does, I suggest digging out the leveler's manual for more detailed instructions than I can give here. If no levelers, make sure you have sufficient blocks to allow you to level the coach at uneven sites.
Make sure slides move in and out smoothly and evenly. Inspect the seals to make sure none are missing or damaged.
Check the roof for obvious deficiencies and damage. Check skylight covers, A/C unit shroud and antenna for damage.
And so much more............
Perhaps the best suggestion I can offer is to find a place close to home to actually camp for a few days. Try everything out in an environment where help and supplies are close and familiar. That way you can also ensure you all the needed "stuff" to make and break camp.
2014 Seneca 37TS
Toads: 2014 Ford Explorer Sport or 2013 Ford Focus ST