If I had to find a cab leak I'd start where the cab meets the roof and the sidewalls. There's a lot going on right there and constant flexing while going down the road stresses the joint.
I'm not saying windows don't leak, just that they are less likely to- 2 flat surfaces joined with butyl tape can last a really long time. I think the caulking people put on the outside is just 'feel good' caulking. It true what they say about silicone- nothing will stick after its used. I checked mine the other day and they were about completely gone.
Of course if you're really serious about finding the leak there are companies who fill your interior with smoke. Not sure but in California I believe if you have running lights they have to work.
We like our cab over space. It's a huge bed and storage area.
