One way to add an exterior tool box to your camper is by simply mounting an extra battery box. The solution is cheap, easy to install, and conveniently accessible.
Here is mine, mounted right alongide the actual battery. Easy to open and close. I store wood blocks that are used to pad my stabilizer jacks.
There are quite a few battery box options available at Amazon, like this Group 24 size vented battery box for only $13.