Bad Master Cylinder is the primary culprit in this case.
At the age of the chassis (1991) I would replace pretty much everything in the brake system. Master Cylinder, hoses, calipers and pads. Rotors too if they have much scoring or warpage.
Do it soon! You are little more than a loose cannon waiting to kill someone with a 15,000 lb cannonball.
Sorry for sounding so serious but it is a serious situation.
1995 Coachmen Santara 360MB 36' w/slide.
Ford F53/460 chassis, 1997 Jeep Grand Cherokee "toad"