I had an interesting conversation the other day with Allen at Flying A Trucks at Cuba Missouri. He is specking out some trucks for the Waco Texas fire department and dealing with 12 ft. high doors in all the old fire houses. The emergency responce people want as much height inside as possible but have all kinds of electronic gear to put on top of the truck boxes. Here are some things that might help:
1)Low profile AC saves 3 or 4 inches
2)Go to 19.5 inch tires instead of 22.5 (maybe some handling problem issues and weight carrying issues, ask warpath for some good information).
3)Consider a low profile truck like the international 4300 or 4400 (GVW issues depending on the weight of your planned box).
4)Some companies will build lower profile boxes I believe. Maybe a 6'6" interior instead of 7' plus.
5)Increase garage door height
I continue to learn alot about these new truck RV's. I am fast coming to the conclusion that there is a market developing for the non-racer truck RV with the emphasis on RV much like warpath is building. My judgement is that 13' plus heights are an issue that must be dealt with to broaden this market and I think there are or will be some solutions. Our current fifth wheel will just clear 12' and I would have to really be careful with a higher rig on the roads and parks that we frequent.
Hope this helps.