You can buy latches that are identical to the locking latch that have a thumb knob on them, you can google cam-locks and find several vendors for such.
Then you just need to drill and mount them on your door in a similar fashion to the current locks you have, this will allow both options.
As for your stuck locks, try some WD40 in the lock, also, put gentle pressure against the door holding it shut while you try turning the key. Also, look at your keys to be sure you are using the correct key, it should be printed on it CH751, which is a standard lock for all trailers. So really the only thing you are doing is keeping an honest person honest, any real theives already know the keys are universal.
While you are searching for cam-locks, you can order new locks as matching key sets that are different than the CH751 keys.