I mounted a solar panel on my 2000 LTV and because it was mounted using 4 aluminum brackets to allow cooling under the panel, rather than flat to the body of the van, 3M advised me to use Mounting/Sealant #4000 which they said is designed to adhere products of different chemical makeup. They were very helpfull with any questions I had. The sealant comes in a tube, is quick to dry, but has terrific sheer and dynamic holding power.
The VHB tape was my first consideration until I talked to 3M and had difficulty finding the tape that I felt would do the job I was doing.
Because of the heat generated by solar on the roof, I wanted ventilation under the panel, hence useing the mounting brackets to lift the panel 1/2 to 3/4 inch off the roof.
Good luck which ever method you use. Don't hesitate to call 3M's toll free number for their advise.
Barry