New Fridge, Old Coach

I recently installed a Norcold Polar 7 fridge in my Mother's older (1991 era) motorhome.   She never uses it, but likes to keep it around and keeps it maintained.    The Norcold works wonderfully on propane, however when the coach is plugged in, it will not run on AC.   If I disconnect the fridge from the outlet (on the outside vent) and plug the fridge directly into shore power via an extension cord, it works fine.   

I am not an electrician by any means, but I am guessing that since the coach shore power is running through the original power converter, that the new Norcold does not like the "dirty power".   The outlet has 120v and I can plug in other things like a fan into the outlet and it works fine.  As I mentioned, I can plug the fridge directly into shore power (bypassing the power converter) and it works exactly right.   

If I am correct on the "dirty power" concept, what is the method of resolving the issue, besides installing a modern power converter?