Does price matter? You can look at things like in bed / bunk / sofa safes like this one: http://bedgunsafe.com/concealed-safes/
You could have a custom one built from aluminum into a bed / under bed or coffee table if you are full timing. I'm sure you can find a local fab guy looking for the work. Wrap in wood and you are in business. It's just for storage so orientation isn't that big of a deal, and the difference between aluminum and steel won't matter, both will stop a grab and go thief. A real thief will just steal the trailer...