For a while my drive belt has been "slapping" the inside of the casing and since my overheating incident in Branson a couple weeks ago it seems to be worse. Every time I take off the belt seems as though it flutters against the casing. Is this indicative of a bad belt (it looked fine) or perhaps there is some sort of tension adjustment on the belt I should look into? Any input would be appreciated.

How long since changed/how many miles on scooter? I believe change interval is 16,000 miles. Lots of miles, pulleys wear.

Belts beat on case seems to hurt nothing. My Helix(s) used to do it after long highway runs, then went away.

Belts seem to flop around in normal operation.

only about 8300 miles on scoot. Never been changed.

Any Swing with stock rollers in the variator experiences varying degrees of belt slap. Some are worse than others.
To eliminate belt slap get either Dr. Pulley sliders in the Honda variator, or get a J. Costa variator. Some guys in Europe use Malossi variators. I can't speak for those.