I had that problem twice in my 89 chevy pickup. Once I had to replace the cruise control module (a gear in it stripped) and once the turn signal lever/cruise control switch. I also have that problem in my new GMC pickup only when pulling my trailer up a hill at highway speeds. Could be a defect or the poor thing is just being overloaded trying to keep all that weight moving uphill at that speed.
I had a 91 Suburban with same problem. Turned out to be the spark plug wire which passed close to the module was arcing to the cruse control module and causing it to release. A new set of wires solved the problem
I had the same problem on my 88 GMC 4x4 and on my Southwind Motorhome with the Workhorse Chasis. The brake switch was mounted in the wrong position and was loose which meant that every time I hit a bump the cruise would kick out. On the motorhome it was so bad that some times the cruise wouldn't set at all. I took the unit to Terry Labonte Chevrolet in Greensboro, NC and they fixed it immediately. The rv dealers didn't have a clue.
I just got through repairing the almost same problem on my 1993 Fleetwood Holiday with a G30 Chevy 350 V8 chassis.
On mine, the turn signal/cruise control stem seemed intermittent... had to "fiddle" with it to get cruise to engage, and then cruise would "fade", especially when pulling hills.
I found a replacement control stem, with all new switches, for less than 40 bucks at: http://www.1aauto.com/1A/Electrical/ChevyTruck/VanFullSize/PLP2310851/40419
This new arm solved the intermittent problem, and it is an easy pop-out push-in replacement. The RV cruise worked better, but would still fade out when pulling hills. I tried to find a replacement actuator assy ( the throttle-pulling assembly mounted on the drivers side just behind the carburetor ) but the only ones I could find were over $300.00 (Friendly local Chevy parts dealer).
I took mine off, just 3 screws , unhook the throttle-puller link wire, a small electrical plug, and 2 vacuum hoses. I carefully disassembled it and didn't find any cracks in the rubber diaphragm or O-ring holding things together. I did note, however, that about 4 chrome-looking wires (that make contact with the diaphragm) were a bit scuffed and corroded. I cleaned them as bright and shiny as I could, and reassembled it carefully. It now works GREAT, and pulls hills without fading, even downshifting when pulling hard enough instead of just lugging down and fading out and dropping out.
Having working cruise control REALLY improves the driveability of the RV... makes it almost a pleasure instead of a white-knuckle endurance experience.