Seam Sealer, on the outside. Recommendation needed

I have this:

Its a Frank Industries  Xplorer 228 on a 85 B350 chassis.

For those not interested in reading my ramblings, any recommendations for seam sealers that will last for 20 years when used on the outside and exposed to the NH weather ?

Last year it started leaking, in the rear bed area. The window channel strips where crusty and the flocking (he,he,he, I said flocking) was pretty much non existent and there was signs of water coming though. I replaced them, the windows slide nice now, there is no water building up in the channel. But I am still getting water intrusion. I will be resealing around the windows also.

To be sure I solve the issue, I will remove the aluminum gutters, and want to reseal where the fiberglass roof meets the steel van body. I was thinking seam sealer will be better than just about anything at LowDepot. I am not removing the roof and back 1/4 of the rig to do this or stripping the interior. So this will be done from the outside. 

The dodge drip channel still exists and thats what the gutters are attached to. The fiberglass roof sits over an unknown amount of original steel above the drip rail. Assuming not much.

Any recommendations for seam sealers that will last for 20 years when used on the outside and exposed to the NH weather ?

Thanks

Steve

Its not seam sealer, but I know 3m windowweld seems to hold up for a decade at least thanks to the uv stabilizers. Can get ugly fast if not carefully applied, and not the cheapest but made for automotive exterior application. Maybe there is a roofing type product that would be similar and cheaper. 

I know that mercedez has their own brand of seam sealer that is supposed to be bad-ass. 

Does the van seal up enough that you could smoke bomb the interior and find the actual leak or are you going with the hailmary  method? 

I have used this stuff and it works...  3M Sealer  Does it work for 20 years?  Don't know, but I have never found / used a bad 3M product.

In reply to NOHOME :

I was going hailmary, but the smoke bomb intrigues me.  The seals on the door windows are toast too, but it's easy to see that they are an issue, but not the cause if the wet bed area.  

I was happy with the Wurth seam sealer I used on my old BMW, it seemed pretty similar to the stuff I was wire wheeling out to do rust repairs and had definitely held up over the 30+ years. 

https://www.wurthusa.com/Chemical-Product/Adhesive-and-Bonding/Seam-Sealer/Brushable/Brushable-Seam-Sealer-1-Quart-Can/p/08901021

Adam