David,
I asked the dog show couple who owned ours the same question. They did not get the winter package, with heated tanks. They did show dogs as far north as Ohio, though, and never had any problem at any time of the year.
The underside of our Holiday Rambler Alumalite is enclosed, so heat that escapes through the floor of the trailer tends to keep the tanks above freezing. So far we have not had a problem either, but we haven't really put it to a severe test either. The RV service guys who did our cosmetic work told me that they didn't think I would have any problem with winter use in our area, near Cincinnati. Our winters are pretty mild, with lows in the twenties mostly, and down to zero a few times. It does get below zero here, but not often.
My advice about the subject is that if winter use is important for you, look for a unit that has the winter package installed. If it doesn't have that and you think it's necessary, a reputable RV service center should be able to retrofit your trailer, but it will be more expensive than ordering the option from the factory. If you will see below freezing temperatures only occasionally, you should be okay.
Loren