Well, I've never built it from the RCM plans, so I can't say.

I put foam in the wing to:

A) Create a wheel well

B) supply support to the sheeting

Without the foam, the green shaded area in the pic below would have no rigidity to it (Note the location of the spar {Brown line} and ribs {Red lines}). That sheeting would just be flappin' in the breeze.

Flap, aileron and retract servo are all in the wing - Retract servo right in front of the spar, Aileron servo right behind the spar, and the flap servo behind the aileron servo (There's plenty of room for this, even on the 40 size)