Ho, you're way out over your skis here.
The truck bed is 2.8 sq. m in area. Assume 20% panel efficiency and 5 hrs/day insolation @ 1 kw/sq.m.
That's just 2.8 KWh per day. Pick any other values you want, just state your assumptions and show your math.
How do you get bty size out of that? You can't. As long as the pack is physically capable of storing the amount of energy captured in a single day, solar power rating
tells you nothing about the vehicle's energy storage capacity.
Now if Elon had said
it takes 35 days to fully charge the SR pack, we'd have the straw we need to make our bricks. That'd make it a 100 kwh bty pack. But as is, the revealed specs tell us nothing about the size of the bty. We can only estimate the avg daily charging rate, and even that given some assumptions.
Cheers!