I don't have a diesel or a scan gauge II, but I've been using a 1st generation scan gauge for years. In an F250 pick up as well as a Coachmen V10 powered motor home. I've used it in my new motor home but it's had issues like turning on when everything is turned off. I don't know if it's the gauge screwing up after all these years, or an incompatibility issue with my new V10. That being said I plan on buying a scan gauge II for the new motor home.
As far as your question on the scan gauge II and a diesel, it will work, depending on the protocol in your MH. If it has the OBDII protocol (it probably does) it should work fine. If it has the J1939 or J1708 protocol I believe you need their scan gauge D