Greg is right in the ballpark. We have a TruckFridge 4 cu.ft. One of things we have noticed with our set up - hot temps outside equal lower solar power generation from the panel than when it is cold.
Based on a year and half of heavy usage, we are really happy with the system we setup. Our 400 Ah plus AGM battery bank can carry us through three plus days of zero power generation from the 300 watt panel. With even a cloudy day the 300 watt panel keeps the 4 cu ft fridge and a 50 Qt. Chest fridge, we use as a freezer, going with no need to plug in or run the engine to charge. The system on normal days reaches float usually by 11 am.
I think 240 to 300 watts is a sweet spot for our rigs. Leaves space on the roof for other stuff and provides enough cushion in power generation for even overcast days. Conservatively, figure 60 to 80 amps of power per day from 300 watts of solar.
Beastie 3: 2002 7.3 EB Cargo: Agile TTB, CCV High Top, Custom Walk Through, Lots of stuff added. www.BlingMyRig.com