I think the very FIRST thing you should do (other than checking that you don't have any water leaks) is to apply screens to all of the outer openings you can find. You can either buy the pre-made screens (more expensive but less work) or you can buy some aluminum screen and cover the openings yourself. Walk around the rig and try to imagine anywhere the little buggers might get in.
I've had 4 TTs and in that time I've had hornets in my refrigerator, inside the propane cover (twice), and even inside the storage cavity where the power cord goes. (Thankfully now the FR just has an external locking hookup for the power cord.) I also had birds build a nest inside the air conditioner cover on the roof!
In some cases you may need to remove a grill or fixture that is caulked into place so you can put a screen in place. Just do it and then re-caulk. On my last 3 trailers putting screens on everything was the very FIRST thing I did after taking delivery.
I think getting the Eternabond and applying to the new roof was the wisest thing I ever did. I was always a bit lazy about roof maintenance and on my last TT that came back to bite me in the form of leaks. On my new rig I decided to be more pro-active and hopefully avoid that fate a second time.
You do need some sort of support materials for the stands, the tongue post, and for leveling under the wheels. I like to use 2x6 lumber pieces myself but these can be heavy. I also own two sets of Lynx levelers and have used them for decades. They're like legos and can be used for just about any leveling application in any combination:
Amazon.com: Tri Lynx 00015 (10) Lynx Leveler: Automotive
The sewer hose support and the screen door upgrades are definite "nice-to-haves" and I would recommend them, but they aren't exactly "need" items.
NOW...one "need" item for me that you didn't mention is some sort of water filter. I prefer an external water filter that I can put up/take down for travel and change easily. My current rig has a built-in water filter but I wonder what moron decided to put it in such a difficult to reach location??? So even though it's there, I'm going to have to remove it or create a by-pass because it's just too hard to get to for winterizing, changing the filter, etc.