Creative idea for sure, just not a realistic idea.
As someone has mentioned, weight is a major drawback.
A truck with 20K of cargo capacity will not be cheap to buy, nor to operate and some places may require commercial registration.
They are also not as strong as one would think, roof is one of the weak points since the containers are designed to handle the weight of other containers on the SIDES (Mythbusters tried burying some shipping containers for a project and less than 1 ft of dirt on top collapsed the roof in).
Containers also sweat in the winter and become hotter than heck in the summer.
Modifying steel does not come cheap, hiring a welder/cutter to make the mods could cost as much as $200 per hr in some areas..
Shipping container doors are not as "secure" as you might think, they are designed real simple and are locked via a standard off the shelf pad lock..
A cheap Harbor Freight $19 battery powered grinder with a steel cutoff wheel will get that container open in less than 30 seconds. Sure, it will make noise, but if you are not around, you will never hear it.
Have seen plenty of posts on other forums which folks bought a shipping container to store expensive things like tractor frontloaders/ATVs or building materials on remote land only to come back to finding an empty container with the lock cut off and on the ground.
Personally, if you are this concerned about someone breaking into your RV, you ARE in the wrong neighborhood to start with. Might be far cheaper and easier to simply find a SAFER neighborhood.