Another Crown conversion nut found in the hot California summers that one 13,500 btu roof air conditioner could not keep his interior comfortable. Now, comfortable is a relative term. For some people an interior in the mid-90's is OK. Not for me. If it is 105 outside, I want my bus interior to be in the 70's. That is why I learned from the earlier conversion on this site and super insulated the space between my roof and my interior headliner, insulated the sides, insulated the floor (in progress this week), putting 3M film on the windows to cut 60%+ of the heat transfer through the windows, and put two 15,000 btu air conditioning units on top. These units also have heat strips for the Winter, plus I have an electric fireplace going in that will also keep the interior toasty in the Winter months.
Every person's tolerance for temperature extremes is different. I am an admitted wimp when it comes to hot weather. I can bundle up when it is cold. But, you can strip off clothing only so far in hot weather before you have no cloths left and it is still too hot (scary thought and visualization).
So, as wmkbaily said earlier, the lessons learned from others is insulate, insulate, insulate. it can certainly cost you. However, the costs will be seem minor if you don't do it and the use of your skoolie is severely limited by unpleasant temperatures (either too cold or too hot).