Interesting--we replaced our back unit[compressor lock-up] a couple years ago and it only seems to run on low. Anyway, there are a series of old-style "dip-switch" settings on each unit that control master Vs slave, which unit controls the propane furnance interface, etc [switches located in the metal electical box on side of unit--once you remove the outside shroud(roof of coach)]. I dont have the factory guide for the setting options but this might be a place to start looking.
2003 40' MDTS
Garden Ridge, Texas