As the owner of a 73
Sovereign center bath (purchased off ebay in 2005 for $2300 that we restored) $11, 500 seems like a very fair price if the shell is in good condition and pics are accurate.
Leaky vista view windows are an unfortunate fact of life and while floor replacement is no fun (partial or full sheet...I've done both), its not that big of a deal. (Obviously very important to re-seal windows).
Based on the market today, you should have little trouble getting your asking price. They're not making any more of them and the Beatrice years produced relatively light weight, towable trailers. We could not replace what we have for less than $25K (many improvements) and probably wouldn't accept less than $30K.
A usable Airstream for less than $12K is a steal IMO, especially with recent plumbing, AC, water heater, etc. replaced.
P.S. - I'm jealous of the Zipdee Awning....those are NOT cheap to add or replace!