Not 100% sure, but according to information that I have found on the Internet... if the chassis is over 14,000 GVW I doubt it has OBD II Compliant signals as those vehicles over 14,000 GVW are not required to be OBD II Compliant.
Quote from http://pureenergysystems.com/store/ScanGauge/FAQ.htm
"QUESTION: What about an RV?
ANSWER: The ScanGaugeII requires the vehicle to be OBDII compatible. Vehicles over 14000 pounds GVW in the US and Canada are exempt from the OBDII requirements. Most RVs are over 14000 GVW. The reason the ScanGaugeII works in most of them is that even though they are over 14000 GVW, they use engines that are from vehicles that are under 14000 GVW. The Ford V8,V10, Powerstroke; the GM V8, workhorse and Duramax; the Dodge V8 and Cummins engines that are found in their pickup trucks are OBDII compatible. Freightliner, MAC, Detroit diesel and the other "big rig" engines are never found in under 14000 GVW vehicles so they were never made OBDII compatible."