There is a glow plug control module somewhere on your rig that may be failing...I have never seen this on my coach, but did have an issue with such a module on a small diesel John Deere tractor.
These modules all drive the indicator lamp separately from the high current supply to the glow plugs themselves, so perhaps your plugs are cycling correctly and the indicator is the only thing that is failing. Here is some generic information on how these modules work, etc.
What engine is in your coach? Start with that vendor for any questions you have on the module, then the chassis manufacturer. Gulf Stream builds the 'house' part of the coach and is the least likely to have an answer for you on this issue..