You are correct, you have to install the tank, receiver, and battery before the servo trays. I built my Knife several months ago but from memory here is what I did.

I shimmed the trays so the servo tops just cleared the hatch and had them ready to be glued in. Next install the radio switch on the wing. Then wrap the receiver and battery in foam and make all the connections to the servos (you may need to run the wires through the tray that holds 3 servos before plugging them into the receiver). Servos dangling on the propper side of the plane and all the connections made, install the radio gear and tank then the servo trays.

You really need 3 hands but it all works out fine.