I have the side by side Elite reefer with the separate compressor for the freezer - pretty standard in the U320 in the 90s. Always had problems running it on propane, always blows out when driving. I pretty much just run it on electric constantly. I added a stack fan and a dual inside fan. When sitting, I have no problem maintaining reefer temps 35-36 degrees.
However, I get on the road traveling over 55 mph and the reefer temp jumps up to between 50 and 60F. No check light. After an overnight we're back to 35F.
Below 50-55 mph and temps seem to stay fine.
I figure either some kind of air flow issue or insulation issue. I know the stack fan is running, I can hear it.
Thoughts?
First guess would be an airflow issue.
How about blocking off some or all of side air intake to see how it affects heating fridge solution?
Think this has been covered in great detail earlier this spring, I would look under the Barry Beam pages, as I think he had similiar issue and found a cure.
Good luck