Another factor to consider is where you are. Northern latitudes receive less DIRECT sunlight due to the angle of the sun. I was surprised by how much less even in August in Homer AK. Having the ability to angle the panels is very helpful (essential) when further up north. The suitcase style portable panels can be angled. While on the subject of sun angle - I find many people do not take into consideration the shading created by other roof mounted accessories including fans. A small amount of shading on a panel (say 5%) can cut the panel output by 95%. try it sometime by covering three cells on a 60 cell panel.
re: sizing - While 200w can be sufficient, I went with nearly double that at 360w. We ski and camp out of our van during the winter. Having more capacity allows us to capture more watts in a shorter period on less sunny days when the sun peeks out from behind the clouds, or more at a lower rate on heavily overcast days, or when the angle of the sun is lower.
Rigid fixed panels can be added after initial installation if you have the room and plan ahead for expansion. Start w 200w keeping in mind adding additional panels should be done with panels matching voltage and wattage and ratings. (Good explanation here:
https://solarpanelsvenue.com/mixing-solar-panels/ )
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