You're probably better off going the route of new external gauges. If you get just the gauges, don't forget to replace with proper engine temp and oil pressure sensor as they might not be calibrated for the old sensors.
I've also seen new fuel gauges, but you have to get those as a kit it'll come with the proper tank sensor that will be needed, but as you say, milage will work also.
You could keep looking on Ebay to find the stock gauges you need, but those will be used and possibly not work well, because well, they're also old and people that restore trucks, cars don't restore MH and I'm sure the gauge clusters are different.
Del & Jerre w/Isis our English Bull Terrier
The Cougar's Den
1984 Fleetwood Prowler "E"