I'm not doubting that things are hard to find but that seems to be a pretty common light on a number of Winnebago, so maybe I can ask where you have looked? There are several different places that "might" have one, so we might be able to suggest other spots to look but that also depends on how far you've looked!
When dealing with old RV in the past, I've found looking at some of the odd little places may turn up odd things back on the shelf!
Checked with Winnebago and Lichtsinn as well as original manufacturer. They don’t make them anymore and none are available. Still checking around hopefully someone has one or can put me on the trail to find a replacement.
I think these may be the same lights as on my '04 Adventurer. A Hamsar 3" round light was what I had. Adventurers in my year actually had two different ones, a fog and a driving light.
I replaced mine with a Hella Optilux 1300. They were a very close match. I think I had to swap the connector. They were not high dollar, probably from the same plant in China that made "Hamsar" versions.
But these days, doubt many folks are even making an incandescent light any more. Probably going to be looking for a 3" round LED something that has a similar mount.