I ordered a 30 gallon Aquatank2 from Amazon. Connecting it to the antifreeze port and using the diverter valve I installed between the pump output and the fresh water tank line, my water pump drains the bladder into the fresh tank without overheating. And that was with the bladder on the ground, using 40' of 1/2" hose.
I wish the Aquatank located the output port on the edge of the bladder. As is, it's on the side about 6" in from the seam. They could strengthen the construction and avoid a seam by folding the material in half and sealing it on 3 edges. The output hose could be attached on the folded edge, reducing the strain on the insertion point. It seems strong enough but some reviews complained of breakage.