You can access the valve by lowering the front bumper and looking straight up into the area above. You could tell by feeling the valve it was not working well since the unit was full of Freon and the compressor was functioning properly.

I would want someone who knows what they are doing to look at it. If they can't find the valve, then I would punt. And you have to remove all the Freon from the system, change the valve out then vac it all down and refill. Not a lot of work, but you have to have one of those snappy reclaimation units. Prevost fixed ours.