Yeah, generally speaking, butt/crimp connections with adhesive shrink are MORE than adequate. The flex that Hookup mentions is the reason to, for the most part, stay away from soldering. "Most" people use too much solder and you get a flex/pivot point where the solder stops and that can cause the wire to break. Even with proper soldering, there will be a flex point - but it is worse with too much solder.
If one is going to use solder, the joint must be physically supported on either end by some mechanical means (cable clamps, for example) to prevent flexing - essentially keeping the joint straight/in-line until after the solder point so the natural flex of the wire can happen.
Joe, I'm not following exactly what you are saying here - can you better explain? "The only thing is I have to use the old shore power plug because it has the factory breaker on my panel . I have to put a female plug on my old charger wire ."
But, why not hardwire the charger directly to a breaker in the panel?
As mentioned, also clarify where you are mounting the charger and whether or not it is appropriate/approved for that area.