Solar battery draining too fast
The battery's capacity is specified at 20h, so 9A. That means that if you pull 9A, its capacity is 180Ah.

Unfortunately, that does not mean you can pull 18A for 10h, or 36A for 5h; for higher currents, the capacity is (much) lower.

You say 1050Wh in 6 hours, so 175W on average. At roughly 11.5V, that means the average current is a little over 15A; well over the 9A at which the capacity is specified.

This is probably where the bulk of your capacity "loss" is. There are also charging and conversion losses, and the cut-off voltage to consider.