Brand new Kodiak trailer - can't use toilet

I presume this is a new rig. If that's the case let the dealer make the repair. If the tank was replaced and this is an older model then, yes, get a whole saw and drill through the tank. Make sure you glue a pipe of equal size to the floor flange so that all waste enters the tank and not elsewhere. You will also have to seal around the pipe so if the tank reaches the full mark there is no seepage.