It appears you are driving a 45' bus, which is probably over 13' high. You need more height in order to walk on the roof of the coach... a minimum of 14' high if the trusses are open(no ceiling), but more like 18' high if there will be a ceiling inside. If your new door is an overhead, you need to allow an additional 2' ceiling height above the door opening... if using sliding barn doors you can ignore this point. Door should be at least 14' wide(it's a lot cheaper now than body shop repairs on your coach later). Rather than raising the roof trusses, it's probably easier, and cheaper to add on an additional 20' x 60' x 14-18' high on one side of the existing barn. My 2 cents.
Kent & Sue & Pecos
2010 Allegro Bus 43QRP
Formerly 2004 Itasca Horizon 40AD