Well Sir,
Mods, adjustments, alterations, enhancements and all that, are all preferences of each owner and, as such, are prioritized as needed. What one of us might do first, based on our priorities, might be on the bottom of the list (if at all) for others. As for myself, if and when I purchase a "new to me" coach, the first thing I do is check all the important systems, i.e. lights, as in brake lights, turn signals and any and all running lights. Then, the brakes. A thorough inspection of all brakes and related components.
Then, the engine. Belts, hoses and anything that I can see, feel, hear or otherwise, needs attention. After all the necessities are taken care of, then comes time for the mods. One of the first "mod" I did was, change almost all of the interior lighting to LED bulbs. Yep, it was a bit expensive but, the interior of our coach is without a doubt, considerably cooler due to that change. And, not only that but, we consume a whole lot less battery.
Our bed was built a bit on the weak side so, I made a support for it underneath the part that tips up. Now, it's as stable as a rock. In the past 7+ years of ownership, there's been plenty more enhancements, alterations and more.
Scott