Exactly so. We have the infrastructure well developed for gasoline and either it is scrapped or duplicated with NG, neither makes much sense.
http://www.google.com/#q=can+you+convert+a+diesel+engine+to+natural+gas&hl=en&sa=X&prmd=imvns&source=univ&tbm=vid&tbo=u&ei=DUGDUP76Beyr2AXSsICQDg&ved=0CGYQqwQ&bav=on.2,or.r_gc.r_pw.r_cp.r_qf.&fp=1ac9a74ee4de457c&bpcl=35466521&biw=924&bih=459 ~ just videos on how to convert a diesel engine to natural gas. There are also two major forms ~ liquid and comparessed ~ but trucks tend to operate in delineated areas so you can build the infrastructure out as the base of demand is converted.