Thanks for the help with this.
No A/C - I resealed the top vent w/ butyl.
I havent yet driven it again in a rainstorm, but here is what I think is happening:
The top vent is sorta old and flimsy, so when I close it all the way, I can still easily lift up one of the back corners and look right through. Would the wind driving down the road at 70mph be enough to lift the vent corner(s) and let in enough water to entirely soak my table bed (which I had set up at the time), as well as the rear part of the cab bed?
I believe it could happen, but I just dont think it happened that way. There was just too much water coming in. I still think it had a lot to do with the water that was just coming in the cab windows I had lowered fully to bring fresh air through so that the exhaust from the cracked manifold didnt smell so bad.
Thanks very much for the help with this - If nothing else, I'll report back with another storm, one which I hope to meet as I leave Maine south near the middle of August.
Also, would one of those MaxxAirII vents be a solution to this instead of replacing the top vent, if it does happen to be too flimsy? It just doesnt keep a strong seal when I close it down. Not sure if the seal shrunk or the plastic grew weak, but I have not problem lifting the cover and peeking in with it fully "shut".