Hi, me again.
For the record, we have aProgressive Industries EMS-PT50X (portable one). It comes with an eyelet where a bicycle cable can be passed through and then the cable can be wrapped around the campground power supply pole and locked. Although any lock is never truly theft proof, it's enough that, if someone really wants it, they'll have to cut the cable somehow. Theft prevention can never 100% fool proof. But a deterrent will go a long, long way.
We have both the Progressive 50 and a Hughe's 30 amp Autofoamer (Watch Dog) that we kept from our previous 30 amp travel trailer. I use the 30 amp Hughes when we are plugged into a 30 amp power supply, and the 50 when plugged into the 50 amp supply. Both have shut down the camper on several occasions in different campgrounds over the years.
The one advantage of using a portable one is, if the protector (EMS) fails, the portable one is easy to unplug and you can just move on.
In my previous camper, it had the camper electric cord that pushed into the mouse hole. However, access was pretty easy on the inside of the camper if I removed a panel. So, I did and put a hinge on it like a door for easy access. I then was able to access the junction box, I cut the wire for the camper and attached a new male end plug I then attached a new female end to the original power cord. I was able to put (my then) Hughes 30 amp autofoamer right there by plugging it in. It was inside the trailer, safe, and could never be stolen. It was still 100% portable in the event of failure and worked great. When we traded the camper for our current Montana, I simply unplugged the protector, and plugged back together the original cord, and kept the protector. It was the best of both this way.
In my current Montana, the power cord is the bayonet 1/4 turn type. I have no clue where the junction box is, all I know is, I've never successfully found it, so I'm assuming it's somewhere under the trailer floor. If not, the wires running from the male socket on the rear of the trailer is wired directly to the converter? I really don't know, and am perfectly OK with the portable versions.