Nobody has answered you yet, so I will chime in.
I can't directly tell you who Rockwood gets their tank heaters from, but there are some choices for afternarket kits.
I was F/T for 2 Alberta winters in my 3 season TT and did some research into the matter. I had a miserable time trying to keep my tanks from freezing.
I forget whose product I was looking at, but it may have been
Ultraheat.com. Check them out, they have a good selection of options for what you are asking. They also have pipe and gate valve heaters.
And yes, do the black tank too. It's no fun having a full tank and trying to melt a poopsicle first to dump it. Been there done that.
