Something to consider is putting on a fiberglass roof...I am considering it on the vintage MH. Fiberglass doesn't leak and last decades, and with proper maintainance..forever.
Not commonly used in RVs, but some use them, they are the most desirable ones to own.
This makes a shuttle bus a good foundation because its roof it solid fiberglass and none or very few holes.
In fact, instead of a roof raise, put thick insulation on the roof glued to the metal and fiberglass over it, and get a couple inches more headroom and more insulation and a roof that doesn't leak and needs almost no maintainance.
Best Class C with fiberglass roof.