The photo is of a selective resistor utilized with the Kelsey Hayes hydraulic brake controllers that were common on towcars when your rig was new. The intensity of the application of the brakes was adjusted by this resistor (unlike our modern electronic controllers that have similar adjustments made via dials or slides on the controller itself). You can read more about the Kelsey Hayes hydraulic controllers as well as the selective resistor (pg. 4 of pdf) in the document linked below:
I am not aware of any modern (recent) controllers that would be compatible with one of these resistors. The last one that I owned was taken out of service in 1983 when its brake fluid displacement exceeded that which was acceptable with my then new GMC G20 Conversion Van.
Good luck with your coach!