RE: LOW COOLANT SENSOR
We have a 1988 AS 290 with a 454 Chev engine. The chassis was manufactured in 1987, so it should be the same as yours.
The overflow bottle is simply a bottle mounted on the left hand side as you are facing the front of the coach that has a "full cold" and a "full hot' mark on the front side with an inlet tube that comes from the radiator cap flange to the bottle and a tube that dumps overboard if the bottle is too full. There is NOT any sensor or wiring to this system.
I am guessing that the sensor for low coolant is probabl mounted in the radiator in an almost impossible place to get to!!!
Good luck, BOB
1988 Classic 290 motorhome