Well, I used regular "gutter and flashing" sealer (from the big orange box store) on the roof of my pickup a year or so back and have no leaks as of yet.
I'll qualify that success by mentioning:
1 - I'm cheap and I already had a couple of tubes on hand...
2 - I'm in New England, so while it seems to stay flexible and adhere fine in the cold and moderately hot summers we have I have no idea if it will stand up to a real hot, Texas-style heat.
3 - You probably don't want to ask about my previous experiments with regular latex painters caulk...
The key seems to clean the surfaces really well and don't try to seal joints on fresh paint - another bit of hard won knowledge, tempered by previous failures.
But then again the real 3M automotive body seam sealer doesn't seem to be that much more expensive.
Automotive Seam Sealer Caulk-Gray, distributed by R.S. Hughes - Industrial Distributor - Tapes, Adhesives, Abrasives, Safety, and Electronic Products
So if the sealer I've got in there now fails in the future I'll probably try the real auto-spec stuff next time.
Best of luck!
-Mike