We go for the simple option. Take both sets of keys with us, and each take a set when we go out. Documents and valuables come with us, and we lock the Van when it is unattended, even if we are just dumping the cassette and/or topping up the fresh water. If anyone breaks in, they can pinch the contents of the fridge, but if they're desperate enough to need the remains of a limp lettuce, a squashy cucmber and 3 mushrooms of dubious vintage, good luck to them ...
Steve