I think his advice is good - especially since you're paying for the upgrades. Why rush into an upgrade that you don't end up needing or wanting?
The upgrades you've identified are good (flush and doors). I'd spend the summer enjoying your TT and identifying things you want upgraded/changed. Then, at the end of the season, take it back and get them added.
Remember, it's just about conveniences. An RV off the lot is just as usable as an upgraded one. Use it for a while and decide if the upgrades will really make a difference for you. I think you'll be fine for the summer without the fantastic fan with where you'll be (assuming you're not staying in the hot part of Colorado) - but it will be something you'll likely want later. I suspect the dealer is trying to keep people's upgrade lists reasonable so the worker can actually complete them.
