NekkidFish - being aluminum frames they can be pryed back pretty easy to pop open the front door lock. Some trailers are probably more secure than others. ALso most trailers use the same coded trailer key which is a blessing sometimes when you have misplaced yours. Just go borrow your neighbors...
My POPUP trailer is real joke hehe. The front door on it has the side fabric walls attached to the door frame using velcro. When I am carrying in supplies it is easier for me to run my hand down the velcro along the wall and reach inside and unlatch the lock instead of putting everything down on the steps and finding my keys hehe...
Of course you can use a knife and cut any and all of that away as well...
We use the OFF-ROAD POPUP on all of the trips and our 5th wheel sets at home as a second home so to speak... We certainly keep everything of any value out of sight and all real valuables are kept secured in the back seat area of the truck.
I have motion lights that come ON under the canopy when you walk up... Always got a scanner going inside listening to Police calls and Ham Radio things all the time too. Probably helps some hehe... Most of the time we are camping off-road and there is nobody around for miles and we very seldom leave the area unattended. At the campgrounds you more or less look out for fellow campers as everyone can tell who belongs and who doesn't...
Things like BATTERIES and PROPANE TANKS are big items the druggie guys like to steal... The 2KW Honda Generator is also a big ticket item thieves like to steal... Ours is permanently mounted in the tail gate corner of our closed up truck bed. It definitely goes where we go...

Just saw a Honda Generator on sale from a Craigs list recently and in the picture you could see where the handle was cut thru where someone had it secured to their trailer using a chain I suspect.
Over the years you find alot of guys rigging up things like dog barking sounds tripped by motion detector - maybe a large dog bowl sitting outside with a chain tied to the trailer wall. Of course the name KILLER painted on the dog bowl... You find signs on the door saying "PLease don't disturb my Dog" etc..
We like having the patio area all lighted up when someone comes up...
I was at ELKMONT in Smoky Mtns Nalt Park one trip a few years back and only my POPUP and one across from it was in this one side road. I left around noon one day to go into Townsend for supplies and got back around 2PM in the afternoon to find I couldn't get down my one way road for a tree being down. So I just drove all around and came up on the other side to get to my site going the wrong way on the one way road.
The neighbor POPUP camper had a big 3KW Honda Generator chained to a small six inch or so tree there near the trailer. Apparently he had left the area about the same time I did and sure enough someone had cut down the tree between noon and 2PM and stole his 3KW Generator. My neighbor came up just behind me and there we both stood looking at the cut down tree in disbelief.. (True Story)
I have been pretty lucky over the years of both TENT camping and POPUP camping... Probably lots of lights and two way radio noises maybe helps ???? who knows...
Common sense things is about all you can do I reckon...
Roy Ken