Gordon - I can't get at the tanks without great difficulty - being the 'arctic package' there is a panel sealing the bottom of the trailer (presumably well insulated behind that) and it appears it would be even more difficult putting it back on. I had hoped there might be someone out there with the same unit who has already figured it out.
K - the grey tanks are always left open to the sewer, therefore with no liquid level within will not freeze. The drain lines are traced and insulated to the sewer. I hesitate to leave a heating element 'ON' with no liquid level in it.
Thanks to both of you for your suggestions!