Hi ourladybug -- Welcome to the forums! Some mention and links for black spots on mousefur are here
. A lot of people report this -- it's usually fine aluminum powder escaping rivet holes and along inner skin seams -- caused by road vibrations.
Window gasketing and parts? In alphabetic order, talk to these folks:
Vintage Trailer Supply
Shipping probably will be less the closer they are to you.
Until the last 5 years or so, spares were optional on two-axle trailers unless they were Classics. Still, a lot of spares are installed under the belly by the front A-frame. Some dealers (or handy owners) could install the factory rack. Your manual does point out that you can tow to the nearest town for tire repair with 3 tires on your coach if you keep it under 45 mph. The easiest way to remove a wheel is to pull the other wheel up onto a single car ramp or stacked 2x8s. I always carry one rhino ramp because it's light and easy. You'll need a bottle jack and lose time if a road hazard ever puts two tires on the same side out of commission. You can only jack on the frame the axles are attached to -- never the axles.