Could be your parking brake switch is not working. Not sure on MH's but on trucks, it lets the ECM know the parking brake is set, which is required for high idle to work. Could also be that your ECM is not programmed for this option. Either issue can be solved at a cummins authorized dealer or any other place that has Cummins Insite on a laptop. We deal with high idle and cruise issues at work a lot. The first thing we do is connect laptop and see what ECM is reading as far as switch positions.
Could also be your service brake switch but if your cruise works going down the road and the brake kicks the cruise off, then it's probably not that.
I use high idle to build air quicker so I'm not the guy sitting there forever with engine running. Get everything else ready. Start engine and gen, go to high idle, unplug shore power, drive away.