We had about 185 watts on our SMB. It was more than enough for our TruckFridge 45 and a few lights. The real issue is the tree cover and amount of shade where you are camped. In full sun the battery would be back up to full power in about two hours. In a shady campsite it took much longer.
On our new rig we got a 150 watt Overland Solar portable system that we can move around to miss the shade. It works well. Just have to keep an eye on its location and the shade.
I am not sure solar is worth it if you are planning on driving every day. If you have a big battery bank and drive every two or three days it may not be worth it either. It all depends on your power loading. An all 12volt rig with all LED lights uses very little power.
Read detailed trip reports, see photos and videos on my travel blog, luinil.com.
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Current van: 2002 Ford E350 extended body camper with Colorado Camper Van pop top and Agile Offroad 4WD conversion.