Ford requires a higher temp brake fluid in the F53 motorhomes. It used to be called "Super DOT-4" but they have revised the name to DOT-5.1 to match common industry nomenclature.
Boiling brake fluid make the pedal soft, but as mentioned it should only be a problem after long braking, not just one hard brake application.
If you want to improve your brakes, the best thing you can do is upgrade your front axle to one for the '08+ E-series. Bigger rotor, caliper, and pads. Its one of the best upgrades I've done to my van. You can pick up a take-off front axle from QuadVan in Portland for $400. As a bonus, you also get new/better ball joints and suspension bushings, plus an improved radius arm and sway bar design.