Walt, The orifice for the reefer burner is very small, so it takes several minutes to get gas up to the igniter. Since the furnace lights you apparently have propane in the tanks. If it doesn't ignite after several attempts you may have something blocking the orifice. Be careful cleaning it as the hole is quite small and can be damaged.
If this doesn't work my suggestion is to Google Norcold and read their trouble shooting guides that will help you diagnose better than I could explain it.
'02 Ultimate Advantage 40J Spartan MM - Cummins ISC
2013 Jeep Rubicon JK Unlimited