Yeah, the hybrids would reduce heat/cooling capabilities to some extent as you'll have canvas instead of all hard walls. But few trailers have quality insulation so winter camping will likely require the same preparations - bigger battery bank, generator for recharging, more advanced charger, etc.
If your state parks offer electrical hookups, then you can bring a small space heater. I keep one in my RV that has a remote control. So in winter if I've got electrical, I just turn it on while I'm in bed and wait for things to warm up - I don't bother running it during the night (have great bedding) but it's nice while getting dressed.
Quality bedding makes a huge difference during the night. You can sleep quite comfortably with the right bedding even in such low temps. And they don't have to cost a ton.
With the kids being 4 and 6, you should be able to get about 4 years with the bunks before they start getting too small for the kids (unless you come from a stocky line :) ). Then you could add a tent for the kids and use the bunks for storage.
Yeah, water capacity can be tough. How the sinks are fed to the waste tanks makes a difference too. Depending on your family's usage, you can fill one waste tank very quickly while the other rarely fills. IMHO, unless you will be using water hookups, you want your fresh water tank capacity to be close to your waste tank capacities combined. Carrying some water in portable containers for drinking and cooking is fine, but having to add a bladder of 20gal to the back of your tow vehicle will reduce your payload and the hitch weight you can handle will be reduced. A bigger fresh water tank in the trailer will already be accounted for in its GVWR.
