Jerryc, this is what I understand about overheating or boiling of fluid
What happens when brake fluid boils? okay, so your going down a hill, its a steep one and instead of using engine braking (very important in a heavy camper) and occasional use of the brakes to slow you down you go all the way down the hill on the brakes, the heat generated by the friction of the brake pads against the brake discs causes heat to build up on the disc and in the caliper, this heat is transferred to the brake fluid.
If the brake fluid is old and has a low boiling point then the brake fluid will begin to boil, when the fluid boils it creates bubbles, so when you next go for the pedal you will find it sinks to the floor with very little if any effect on retarding the vehicles speed. Not good, once cooled the brakes return to normal. This is know as brake fade, vaporized or vapor locked depending who you talk to.