The 2x4 and fork mount is the best option. In my trailer I had a place to bolt to the floor, but I used unistrut instead so I could adjust the fork mount.
If no place to bolt down, then maybe no need to, just make the 2x4 long enough that they dont tip over.
Or, easiest way is to use moving blankets and lean bikes on something. Taking off front wheels helps to lower them, keep them from rolling and is easier to lean on each other (because yo can turn handlebars 90 degrees with wheel off and makes them flat).
I have used all three methods.
You will always be dealing with bikes in the way unless camper is just too big and yo have spare room.
Finding a way to haul them outside is best. A front receiver is great. I sometimes haul mine inside, then put on outside bike rack while I am there, Inside is more secure if you leave it unattended and inside is dry.
Do not DO NOT use the braided cables to lock them. These can be cut with scissors and every homeboy carries tools to cut these cables. Get the biggest, baddest chain or locking device you can.