hi dennis,
this one is a "up to you" kind of answer. i have had my 1990 coach since the summer of 2007. it has the manual leveling system that is a series of toggle switches on the center console. many others have the HWH control system. having said that, i believe at the air control block under the front bumper and on the back engine firewall that they are the same. they use a valve that is made by HWH and is readily available (though they are 80 bucks apiece). i have replaced 3 of them so far and keep a spare. they do seem to get weak over time and need to be replaced, though my experience has only been the past year. i dont believe that 3 a year is the failure rate as many of mine were old.
so, if you are positively sure you cannot find the valves you have, then you may be stuck. but there are many members here who have 1989 and back coaches that may be able to help you source those solenoid valves.
as far as i can tell, the air manifold that the valves screw into is just a block of aluminum and the cost of converting just that block would be the price of it, the price of the new solenoid valves and if you had to redo any fittings onto the manifold. but, i could be wrong.
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