Soldering is a bad idea in an application (such as RVs) where things move.
Electrical background from the USAF, the one thing that was drilled in to our heads was "soldering isn't a preferred practice on wires in a vehicular application".
Heat shrink crimp connectors are fine.
With regard to your trailer pigtail, does it have movement (side to side) in the truck's plug?
It's a known fact that there are some trailer pigtail manufacturers that don't build their plugs to the exact spec (in diameter) and they fit loosely in the OEM truck receptacle.
If the plug has any slop in the truck's receptacle, that's the most likely source of the problem