Do they all lie on a common plane? You could start a sketch on one of them, convert all letters, then extrude.
Then you could tidy by using delete body on the surfaces.
*edit* you can also use surface extrude command, click on the surfaces (or alt + click if planar), specify the direction and end condition.
Yeh both of These worked for me. I don't like using delete bodies much so extruding Surface and knitting was "cleaner".
However it does seem silly that thicken wouldn't work with multiple planar surfaces.