If I remember, there's a voltage regulator ON the back of the Cluster. It may have failed. At least find it, pull it, and clean its contacts to the Cluster.
Also, try to figure out where the Cluster itself is Grounded and verify it's good.
There's a recent post here about false over temp readings, and it was a cluster problem. Look through recent activity here in Class C and you should be able to find it. That OP might have suggestions.
I know the Ammeter in our 1983 E350 went out. I didn't research it. Installed a voltmeter and relied on that. I don't think it uses that Regulator, but if I recall, the three you mention (Fuel and Temp) do.