[vettestr]

Thinking about the thread more it came to me that you propably have an isolator now that removes power to trailer when tow rig is not running. Thus the problem is how to get power when parked +++++.
Add a tow rig battery and pull from it or add trailer batteries (that is the best) and pull from them. When they are low you can start tow rig to charge the trailer. If you defeat the isolator bad things will happen sooner or later as a rule. New cars with computers do not like voltage hits and sags so fight temptation to defeat the isolator. buy more batteries as cheaper than repair costs to tow rig.