Fastening Flexible Solar Panels to a trailer-what adhesive to use ?

VHB will work.

I installed four 100 watt panels on my motorhome using polyurethane caulk to mount 1-1/2" aluminum angle on the short dimension. Panels are bolted to that so there is no roof penetration. That was 13 years ago and they are still solid.

Used the same method for mounting a panel on my Cobra aluminum top. Note this is a rigid, glass glazed panel which would be exposed to far more stress than a flexible panel. The mount is quite permanent, so I prefer not to mount the panel directly in case it needs to be replaced at some point.