We had a Suner Power 20w trickle charger from Amazon before installing
a 400i-like solar system. Started out mounting it on the hitch and went thru the truck connector with a std male plug. Eventually moved it to the rear AC and used the solar roof gland. It worked to keep a pair of deep cycle batteries charged while stored in SoCal.
You might need more watts depending on how often it’s sunny in your area.