It would appear as if you have a pretty good foundation on which to build your tiny home.
In regards to your top speed and slowing down on the hills, it all is determined by which engine, transmission, and rear end gearing you have.
With a 190 HP engine it is going to struggle to go highway speed or go up any hill.
With a 4-speed Allison AT540 or MT640X transmission you have no overdrive so your top speed is limited by your rear gears.
Most school buses of that vintage with diesel engines have rear gearing to limit top speed to about 57 MPH.
Your options to change those parameters are only limited to how much $$$ you are willing to spend.
Most diesel engines have a little room to increase HP. If you have the T444 engine without a turbo the easiest way to increase is to get a Banks turbo kit. The engine is essentially the same as the 6.9/7.3L. Banks developed the turbo system that IHC ended up installing at the factory.
Whether your engine already has a turbo or not any HP increases will come from adjusting the timing and the pump. Again, a lot of information that applies to diesel engines in Ford light duty trucks will apply to your engine. It also may require changine injectors.
Increasing your HP will help on the hills but won't change your top speed. You can only change your top speed in two ways--increase your engine speed or change rear end ratios.
Increasing your engine speed by changing the governor setting is a good way in which to blow your engine up. So I would not suggest that as a good alternative.
There are two ways in which to change your rear end ratio. The easiest is to put taller tires on the rear. If you currently have 10R22.5 tires going to 11R22.5 or 12R22.5/11R24.5 will increase the height of your tires and change your rear end ratio.
The only other way is to actually change the gears in the rear end. Somewhere on the rear end there should be a little metal tag that will give you the rear end ratio. If that has disappeared you can raise one of the rear wheels and count how many times the drive shaft turns for each full revolution of the tire. At a guess you have gearing right around 5:1.
If you PM me your VIN and e-mail I might be able to get the build sheet for the chassis for your bus. It will tell you exactly what was built into the bus and sometimes even specifies which rear end ratio was used.
When my mother-in-law passed away very unexpectedly my wife had to go to Grand Rapids, MI to take care of everything. In order to get two of her mother's cats and a lot of "treasures" back to WA state we purchased a 1984 IHC/Thomas bus with a non-turbo, non-electronic T444 engine and Allison AT543 transmission. It was geared to a top speed of 65 MPH but quickly ran out of steam on any hill.
Drafting behind semi-trucks really showed how much wind resistance had an impact on how hard the engine had to work. While drafting it was easy to maintain 65 MPH with about 3/4-throttle. Once out of the draft it was pedal to the floor to maintain 65 MPH.
Changing gear ratios to get the engine speed down at a particular speed won't necessarily also result in lower fuel consumption. Speed and wind resistance are the two biggest fuel users in a bus. Reduce either and fuel consumption goes down remarkedly.
Changing gear ratios to get more speed without changing anything else will usually result in slower speeds up hills. Or at least require downshifting sooner.
Since you indicated you had to replace all of the belts before you headed home I would guess it has been quite a while since any other maintenance has been done. Clogged fuel and air filters will really slow you down as well. You won't notice it under 35 MPH unless they are really clogged. But at highway speeds or climbing hills when you need the maximum amount of air or fuel and you will discover even a little bit of clog will drop your speed on a hill noticeably.
Good luck and happy trails to you and your family.