I encountered that problem on a DP I used to own. The last 25 or 30 gallons had to be dribbled in. The problem was the angle of the filler pipe from the outside outlet, to the tank, it was I guess too shallow a drop. A tech straightened the filler out some, and it helped, but for the eleven years I owned it, filling it full was always a PITA. If I was willing to settle for 3/4 full it was good.
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