Power Supplies For 12v only fridges

at moment i have a waeco fridge using a mobitronics rectifier to power it when on 240v power. Being somewhat of a cheapskate at the moment I dont wish to throw away my el cheapo GMC 750w 2 stroke generator. The problem is the fridge wont run off the gennie using the mobitronics rectifier, it keeps bringing up the alarm for low voltage. Is there any other power supply (not charger) that I could use in place of the mobitronics that would not be so sensitive to the variations of the gennie?. The one that must be obeyed does not want to go into any large costs for smartchargers, etc nor does she nor I want to have to run gennie for long periods of time. I was thinking if I could get a suitable power supply, i would then be able to use my existing 3 stage charger(6 amp) to pump all into the battery whilst not under load. Any help appreciated.