I have a slightly different problem with my '86 Camper GL.

My coolant light comes on every now and again although the coolant 
temperature is, evidently, perfectly normal.  The coolant reservoir is full, 
even over the tube to the overflow tank.

When I release the coolant reservoir cap, pressure is released.  When I 
screw it back on, the problem is gone and does not recur for maybe two 
months.

The line to the overflow tank does not seem to be blocked.  The problem 
seems to happen most frequently when I enter the freeway before the engine 
is completely warmed up, and it reaches its normal operating temperature 
when I'm going 60 mph or so with the space heater on (although it has 
happened otherwise once).

I lose virtually no coolant over extended periods of time.  I've not added a 
total of a quart in over a year.

Now if I could just find a CHEAP (but reasonably conscientious) place in the 
Washington, D.C., area (preferably Virginia suburbs) for some relatively 
minor body work, I'd be a happy camper.  I can't believe they want almost 
$500 to fix that little ding!

Woody