Not sure it is necessary to go to the expense of a diesel heater if you have electricity. If you can afford the space, a smallest size (usually three column) column heater may be worthwhile as you could safely leave it on during the day while you are out - I wouldn't do the same with other heating types except an air conditioner. I would put greater faith in a heater keeping you warm than an air conditioner. Then also have a fan heater to really jack up the temp when you are home. If really worried about a power outage, get one of those butane space heaters - seriously effective but watch the kiddies around it, and don't expect to use it.
Yes to electric blankets. They are cheap and effective and don't take much space packed flat on bed. They also draw tiny amounts of power so can be run off an inverter if you do get a power outage. Especially important where you are sleeping above the campers storage space, as that becomes a big cold store in winter so having the heat underneath you is magic. Don't hesitate to turn them on very early of a night as they contribute to heating the space. Consider electric blankets for the kids as well - which may work better if you don't have stretchers. Stretchers allow cold air to pool underneath.
If the kids can share without murdering each other, consider making a big queen bed by putting inflatables next to each other, blanket on that to hold the heat, then electric blanket and flanny sheets and quilt and blanket over the top. It isn't necessary to spend on sleeping bags they may never use again. A long line decent quality t-shirt under pjs works a treat rather than multiple pairs of thermals for day and night wear.
Yes to doing whatever it takes to block major air gaps in the camper. Also consider putting "stuff" between the kids and the camper walls to act as insulation especially around their heads. Bags of clothing (dirty is fine),towels, jackets etc.
Where will you be spending your evenings and cooking? Even if you have a fully enclosed awning, the cook is going to be tired and the awning will be cold, so plan on eating out more than normal.
You'll have a ball!
