What kind of adhesive/sealant to use for my rain gutter?

My guess is that you will want to use Dicor Lap Sealant. Tons of information on Google.

Do you have any fasteners holding the rain gutter on or only sealant? Is the rain gutter plastic or rubber? If no mechanical fasteners I would use an adhesive like 3M 5200 or 4200 if you have screws every 12 ins or so the butyl tape is fine.

Explorer has modified the Dodge roof gutters in the front and rear and these areas become problem areas before the rest of the roof gutters cause closer inspection and reveal the fiberglass has been hiding a worse rust issue for quite some time

Originally roof gutters were sealed with a seam sealer which remains somewhat soft.

The silicone was likely a much later attempt to seal the zone where fiberglass meets metal.

Is there a bunch of rust in the roof gutter?

The 2 part seam sealer of choice by collision chops is a 3m product. the 1 part seam sealer products are no good according to reports.

http://3mcollision.com/products/sealants

If there is rust, which is likely, get some Ospho and treat it as best as possible. Paint or other products adhere very well to a surface treated with Ospho when directions are followed.

While it might appear you have removed all silicone, it likely is still there microscopically and will repel all attempts to adhere more sealant/ adhesive to it.

A Scrubbie pad and Ospho can remove it for secondary bonding. the Oshpo did not hurt my painted surfaces, unless I let is stay on the paint for a while without wiping it off. Even then it just turned oyster white paint a lighter shade of white.

I've spent way too much time on my roof gutters, all because I halfassed it originally, and because the conversion company originally decided regular drywall screws were perfectly acceptable, may they rot in hades.

I agree, complete removal of all silicone, and appropriate preparation procedures and, the use of 3M Sealant/Adhesive products is what I would do as well.
Also, I'm all for getting rid of everyone who is too ignorant to understand how stupid it is to use iron screws.

I've done my Xplorer 230XL gutters using Geocel Proflex RV sealant;

http://www.amazon.com/gp/product/B004RCP49O/ref=as_li_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=B004RCP49O&linkCode=as2&tag=truckcamper-20&linkId=I4DQYZC5DVHOB5BW

I hope I've fixed them well. They are several older threads on here about this, but I'm having trouble finding them now.

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The gutter is a metal "L" shaped rail that gets riveted to a long "U" shaped ridge which is part of the van. Because of the rivets I guess the silicone doesn't need to be that strong of an adhesive. I already have butyl rope and "Dynaflex 230 Window, Door and Trim Sealant". Could I just lay down the butyl rope first and then put the Dynaflex on top? Or would I be better off going with something else entirely?
Landyaht318, yea there's some rust towards the back and towards the front- I'll try out the Ospho. Which one of those 3m products specifically would you recommend? Thanks.
Cloud Dancer- I should change those old iron screws out huh?