There are several RV Junk yards (UR, Recyclers) and one of them may be able to help you.
Else, there are several companies, of which J.C.Whitney is perhaps the best known, who sell new guages and insturments of every sort, including but not limited to Oil Pressure (And temperture) Water temperture (Engine coolant Temp) Tacometers, Transmission temp, battery current (Ammeters) and voltage, speedometers of assorted types, and more. Basically any guage you want, they got, new and in the box, for a price
Finally, if your ride is a diesel (mine is gas) try http://www.simply-smarter.com/
They make an interesting insturment panel