Great to see your participation on the forum. Welcome.
Does you instrument cluster look like the one I attached below? ...I think that one is from a 1988 Allegro.
If so, it's from a 1975'ish ('71-'79?) Chevy P-series step van and some G-series Chevy vans.
Here's a used one for $500 on ebay:
1971-1979 CHEVROLET GMC G10 G20 G30 P10 P20 P30 USED INSTRUMENT CLUSTER 25046469
As far as other other "parts" go, as has been mentioned, most if not all parts can be found by either identifying the specific manufacturer or will be generic parts that have application to many vans, trucks, or RVs.
There won't --or shouldn't-- that many parts that will be proprietary to Tiffin but if you do need such parts, check the salvage yards. If you need chassis parts, of course, GM or Chevy parts will be what you'd be looking for.
Here's a well known salvage yard (one of many across the country) but this one is well known and has a pretty good inventory of salvage units and that is Visone RV in Kentucky. They used to be able to track down some parts for you or keep you on a list if a specific part is needed and they happen to come across what you might need or what you're looking for. I'm not sure if they still do that or not but you can contact them to see.
Here's their Tiffin page but I don't see an older (vintage) unit listed in current stock but you might keep checking this page and/or contact them as they may know where to find the part you need:
Visone RV -- Tiffin Allegro salvage page
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