A little background....my trailer batteries are several years old and no longer good, they won't hold a charge.  I know they are dead because when the trailer is not connected I have zero power.  We don't camp but do use the trailer at about 25 day shows a year, we run the AC at about 90% of the shows so I decided it really wasn't necessary to replace the batteries. 

This weekend we were at a show and it was a cool morning so we didn't need the AC right away and therefore no generator.  We were sitting outside and my daughter turned on the radio on the trailer, no power and no batteries so obviously it was running off of the pickup battery.  My questions are these; if I do decide to replace the batteries how does the trailer know if it is to run off of the trailer batteries or the pickup batteries?  If I don't replace the batteries what is the chances I would run down my batteries (diesel so I have two) by running the trailer off of the pickup?  The only 12 volt things that we would run would be the radio and pump, which would just be for the toliet so it would probably run less than 5 minutes all day.

Mike

The Superduty Fords cut off the power from the truck batteries if the key is in the off position.  I don't remember what you drive, But it could be an inconvences to have to go turn on the truck key to get the pump to flush the toilet.

The problem with turning on the truck key, is that now you have all the truck accessories powered up beside the radio in the trailer.

The radio or running the pump for a few moments is not a big drain on the truck batteries. But it sure would be a pain to load up at the end of the day and find your diesel won't start because it's batteries are too weak.

I have a Dodge but previously did have a Ford so I understand what you are saying.  The Dodge is different than the Ford, with the key removed the 12v will still work on the trailer.  Would running the radio for 10 hours be a drain on the pickup?

Long Long time ago when I was a young man and framing homes for a living, We used to back the pick into the job site. Swing the doors wide open and crank up the radio and play it all day as we worked. Granted we probably started and drove the truck for lunch.  It seemed to be a common practice among construction workers of the time. and I don't remember many dead batteries. And that was with a single battery on a gas engine.

But the other side of the coin is my neighbor who uses his truck to tow his boat, usually on saturdays.  He always has a dead battery. The dealer after checking it out, said he needs to drive it more often to keep the battery charged as all the new electronics that are part of new technology drain his battery just sitting in the garage.

You will just have to try it, Go prepared to get a jump if it dies.