Although I have idea where in Canada 3LittleDucks is, I can relate my experience in doing the same thing in my C over many years.
I too blow the lines clear, the key is not 'power' of the compressor, almost any will make 100+ psi! it's the reservoir tank size. You are basically letting a line run wide open (albeit at only 50 or 60 psi) so you need a large reserve of pressurized air or the pressure will quickly drop to level that will leave water behind.
My FW tanks are inside (under the dinette seats) so I can fill them the day before I leave and just leave the heat turned on.
We flush the toilet with windshield washer fluid. This brings two points though. First, even if the temps are far below freezing, any liquid in the black or grey tanks is NOT sitting still it's sloshing all over as you drive. This dramatically lowers the point at which it will freeze. Secondly, it takes quite a while at near freezing (few degrees below) for liquid to chill to the freezing point and start to solidify, and MUCH longer and colder to freeze hard enough to do anything, especially in something as big as the drain pipes.
I've driven 12 hours in freezing temps with unprotected full black & grey tanks and got home and pulled the plug and they drained no problem.
