These discussions get so much bad info, it really gets hard to make sense of things, so doing your own thinking is really worth the effort as a way to avoid the goofy ideas put out!
So turning on the running lights would sound like one way to control the voltage to the tow but with the question of how much current that circuit can handle before tripping the protection.
But then it may be worthwhile to check how your RV acts with the wiring at the trailer connector. It may be fused and wired as you need since it is designed for the job.
I always like to check when I see things that look like they are designed for what I want but much of the time the folks who design and build things DO actually know what they are doing!
Amazing that folks who do this work all the time seem to know more than most of us who only do it once or twice in a lifetime??
So that brings a question? If the standard wiring used by lots of RV builders has a pin 4 that is meant for charging, is there some advantage to using pin 3 instead? Does your pin 4 go dead when the engine is off or does it stay hot?
If it is connected to stay hot only when the RV engine is running, I would first look at doing the wiring as the builder's designed, rather than develop a "new idea"!
When I find a plan that is industry standard and likely drawn up by full time electrical engineers, I often check that mine actually works as designed but I hardly ever try to improve what the industry says is the right way!
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