I just purchased my first trailer that will need a charge line ( my last two had solar and 110 ac charging) and I noticed that the charge line in my truck has no relay that I can find. This concerns me because if the grids turn on they will pull current from the batteries including the trailer when plugged in. Being that the grids pull 100 amps each and the alternator will not even supply what is needed for one grid at idle. I my have heard wrong but I heard that our trucks had one just for this reason and so you cant draw from the truck batteries when parked and draining them down. I found the relay for the clearance / running lights but none for the charge line. Has anyone pulling a travel trailer had any problem with this or is there a relay I missed. 

This should help.

The trailer power line is a direct connect lead. No relay and is hot all the time. Will not hurt the trailer there is no way for the grid heaters to pull that kind of load back out of the trailer. The line is fused with 40 amp fuse. 

 

If you're not going to do anything fancy...buy some  "7 strand trailer" wire by the foot and junction box like this  https://www.ebay.com/i/272940261668?chn=ps     Wire color is probably same as what your RV has, so already color coded for you.  But double check.

 

I put in a solenoid or an elec one way check valve as a safe guard because when our old convertor went bad it also took out some 12 volt fuses  Even though blown fuses did their job, I still wanted to make sure any elec damage is confined to RV side if it ever happens again. 

 

If your going to get fancy and run other things such as 12 volt supply line for other things such as air compressor, charge line for batteries in pick up bed, a charging station for  my Ryobi tools, 12 volt water pump to refill water from creeks or campgrounds with a water hose bib in middle of campground loop.  I ran larger gauge wires allowing for voltage drop.  Especially elec brake line. On my landing leg I come directly off RV battery with 20 amp fuse and added extra wire so I can connect to pick up side if rv batteries are real low.  (Low batteries as in wife runs heater all night)

Thanks guys. I guess I will be installing a switched charge line. I like the idea of using the 7 strand wire and will start looking for it. I was thinking about a 3 wire cable but 7 wire sounds better because I want to run a larger brake wire also. It may be newer trucks that have the switched line.