I just converted one of my son's newer Bachman DC engines to DCC, it is fairly straightforward.

If you had photos that would be helpful, but here are my suggestions:

The black wires from the trucks need to be tied together with the black wire of the decoder. The red wire of the decoder needs to either be tied to the chassis or you need to isolate the chassis from the trucks and run wires from the trucks (as you mentioned)

If you make sure the decoder and wiring is electrically isolated from the chassis you should be able to simply drill a hole in the chassis in a convenient location and solder your red decoder wire to it.

You will then need to run a black wire from the motor to the decoder (usually the gray wire). I'm assuming there is a red wire from the chassis to the motor, if so, you'll need to disconnect it from the chassis and connect it to the decoder (usually the orange wire).

The remaining wires will be for lighting effects/sound/etc.