So normally at camp with my electric car, I have a cord from the campsite post (ALWAYS a TT-30 here in Canada, I've never seen a NEMA 14-50 despite them theoretically being "the standard" for campsites) Running to a converter box, and then running to my charge cord for the car, then I have a separate extension cord from one of the 120v 15A plugs to a 12V power supply sitting on the ground under the trailer, and then a cord from that power supply to the 12V input on the tongue of the trailer.
In short, it's a mess!
I figured there just had to be a better way! so here it is!
On the outside of the trailer I have 2 ports, "in" and "out" These are both 30A 120V twistlock connectors. (male input, female output)
Inside the trailer I have some heavy duty wire between the 2, and then a smaller wire spiced off which goes to a power supply
This is a 12V 7A power supply mounted under the double bed, and behind the drawers in some waste space. Input comes from the ports on the side of the trailer, and output goes to a relay by the fuse panel at the front of the trailer. When plugged in to shore power, the relay activates and cuts off power from the vehicle connectors, and instead takes power from this supply to power the interior lights, USB outlets, fridge, and roof vent fan.
Now when I plug in at camp it's simply one cord from the post to the trailer "in" and one from trailer "out" through an adapter box, and to my car charger. Much cleaner looking, much simpler, and I don't have electronics sitting on the ground waiting to get wet....