I don't have experience with tank heaters, but I do have experience camping in a trailer in as cold as -10f weather.
On my trailer, I moved all the plumbing inside, which gives me the ability to leave the trailer wet in situations where it's just a bit below freezing. If it's too cold though, I still end up dry camping. The risk of frozen PEX, pumps and toilets is too high for me.
When it's just a bit below freezing, I keep the grey/black waste from freezing by keeping a balance between RV antifreeze and waste. If it's down closer to zero F, I dump my dish water in the snow and use the campground outhouses. At that temp, no fly's, no smell.
On my shiny new 'B', I'm not sure what I'll do. The fresh tank is inside and the van is somewhat insulated, but I'm not confident that the deeply hidden fresh water plumbing buried behind the cabinets and bath is going to stay warm enough on really cold nights.