I just drove from central WI to southern MI to visit two RV (and more) surplus yards. Both have tons of windows, compartment doors, ladders, folding steps, and a variety of other things at very reasonable prices. Madison Central Wholesale (Adrian, MI) also has acres of holding tanks, toy hauler ramp/doors, and random stuff. But the big score was at Tri-State Surplus (Hudson, MI) Tri State Surplus -- Discount RV, Camper, & Trailer Parts!
, where windows were about $20 less (each) than at Madison Central, and where we picked up a refurbished refrigerator, oven/range, and water heater, as well as a new water pump, filters...a whole lot of stuff. And they take 10% off their on-line prices if you buy it in person. Caveat - they are currently only open on Saturdays! Ed and his family were very helpful and kind, and were excited about our project. I highly recommend them to anyone who is looking for RV parts/accessories. And between the 10% reduction in cost for buying in person and saving ~$100 in shipping for each window (we bought 11), the cost savings of going there can pay for a several day road trip to get there and home again.