You can use $game_variables[X] where X is the ID to refer to variables. And you need to use following syntax for the draw_text:
self.contents.draw_text(x, y, w, h, string, align)
(but you know that already.)
You should also be careful with strings. Numbers are not strings until you "convert" them to such with .to_s
For example:
num = 5
self.contents.draw_text(0, 0, 320, 32, 3.to_s + " - 1 is " + num.to_s)
You will get as output
3 - 1 is 5
But using
num = 5
self.contents.draw_text(0, 0, 320, 32, 2 + " - 1 is " + num)
will result in the same typeerror you just got.