Indeed they are not, I have been caught out by that. The definition literally of a rodent is something with big teeth that can eat through plastic.
There is a reason why people store animal feed in steel bins and drums! I’m not entirely sure that my treasured aluminium zarges boxes would be completely resistant to rodent gnashers.
As others have suggested, making sure the contents are as dry and clean as possible would help reduce the attraction but any animal that chews electric cables for fun is difficult to second guess.
A friend had the canvas of an expensive 4x4 rooftent nibbled while being stored in his garage so I am a bit paranoid but condensation and mould are probably more serious concerns. I store a lot of canvas and woolen camping gear (way too much to box up) overwinter in an insulated outdoor building and keep a dehumidifier running 24/7 there are numerous mouse traps dotted around.