I did a post on this topic last year, or maybe the year before, along with my solution. There is no pickup hose inside the tank. The issue is that the outlet is on the side of the tank near the bottom. It isn't on the bottom. So, any water in the "sagging" belly of the tank isn't picked up, and the pump can only pull the water as long as that outlet opening is covered with enough water to not suck air.
I wasn't easily able to explain my solution in my post, but did provide some pictures. Anyway, If you get clear vinyl tubing that fits snuggly inside the fitting/elbow that threads into the outlet on the tank, you can cut it to a length that puts it down to the center of the tank where it tends to sag. That allows you to suck up most of the water. The fresh water dump valve is also not actually down in the center of the tank, so even after dumping there is still water in the tank. I added another port in the center bottom of the tank that I can open and completely dump the tank at the end of the season. I find that with my "intake tube" in place, I end up dumping about 3/4 of a gallon or so that couldn't get sucked up by the pump.
Most people wouldn't even bother with this level of OCD, but I like these types of projects since I retired and have the time.