Randy,
Based on your intent to only use the inverter while driving to power the reefer a 1000 watt inverter should do the job. You might want to consider a 2000 watt as the price will not be much different and you may find you want more capability in the future.
I would also install a pair of 6 volt golf cart batteries hooked in parallel. NOCO makes some nice battery boxes.
https://www.amazon.com/NOCO-HM426-Co...2-8997e41410bb