From what i see from your code "campo" is not the name of the JTextField, but rather the name of the string. If you need to change the String you could say campo = new String("string"); or campo = "string"; if you need to change the text of a text field, be sure that you are calling the correct thing.
I agree, You should either put these JTextFields in an array or even better, in a List. I would do the following:
that code is not meant to compile, but send you in the right direction. this is a much better way of going about accessing multiple Jtextfields. The arraylist has advantages over a standard array because you need not specify how big it needs to be upon creation.