I agree with Down the avenue. Your trailer batteries are being charged while traveling and will provide more then enough energy over the 8-10 hour period to keep the batteries charged. The Frig. won't cycle often while traveling since it's not being opened. When it does it will draw more then the truck is providing but the average energy consumed over a longer time is still less then the truck is providing.
Whether more batteries would help is mainly a function of how often and for how long you will run without being plugged into AC. People generally add batteries to lengthen the time between turning on the generator. More batteries equal longer time between generator on times.
I think if you search this foru or IRV2, with your model number, you'll find a frig. that will fit.