Thank you all for the replies everyone.
Azdryheat - the previous owner did not have the info, and did not use the unit during the winter, so just winterized everything and never used the tank heaters. And I think he picked it up on a repo, so nothing from original owner.
Newman - do you mean that the BLACK waste tank has two mats - one in tank and one on valve? I never considered the possibility that there are no heaters in the grey tanks, and two in the black tanks. But if I knew that was the case, I would just turn both switches on. There is nothing in the galley that operates tank heaters unless it is something I have yet to stumble across...
Farmboy and Garden3, we have skirted the trailer and before I did that I wrapped all the lines with bubble-wrap insulation (and tracing on the main header outside the skirting) from the black tank drain valve to the sewer.
Still holding out in case someone else with the same unit (they had to have built quite a few, we like it a lot) can come up with the answer.
Cheers!