Are you reading those percentages from an in-the-van monitor, or directly from the gauge on the tank? The remote monitors (when they work at all) have limited resolution--they tend to "jump" from one value to the next in rather large steps. So, your extrapolation may be misleading. The physical dial on the tank is more accurate. 54% should be plenty for four days of fridge use, I would think.
Formerly: 2005 Airstream Interstate (Sprinter 2500 T1N)
Now!: 2014 Great West Vans Legend SE (Sprinter 3500 NCV3 I4)