I do agree, snow/ice stuck to the car changes the aero drag a good bit and pushing slush definitely adds a lot of tire drag as well. This can definitely be a major factor.
I live in an area where it can get pretty cold, but usually in quick cold snaps and rarely a lot of snow on the ground. I've found preheating when plugged in completely negated the extra energy needs on shorter drives, but the longer 100mi highway drives still ended up needing a lot of energy to keep the cabin warm. And yes, it takes a ton of energy to go from a freezing cold cabin to warm compared to trying to keep things warm. Every time you stop for a few hours and then go again without having any external power to preheat you use a lot of energy.
I live in an area where it can get pretty cold, but usually in quick cold snaps and rarely a lot of snow on the ground. I've found preheating when plugged in completely negated the extra energy needs on shorter drives, but the longer 100mi highway drives still ended up needing a lot of energy to keep the cabin warm. And yes, it takes a ton of energy to go from a freezing cold cabin to warm compared to trying to keep things warm. Every time you stop for a few hours and then go again without having any external power to preheat you use a lot of energy.