The white bottle Power Service should help that diesel from gelling, although when it does start to get cold a topping off with #1 diesel will help.
I wouldn't put anything else in the tank.
One thing I have always done, since I have mostly lived on the northern great plains in the Dakotas where cold is king, is change the diesel fuel filters every fall. That way I have fresh filters for the coldest winter months.
Also be sure to check out the battery condition and make sure both are in top notch condition.
Those things done, you should be good to go.