Hi,
\While a emergency generator will run it's block heater all year long (yes keep that engine at 100F or so in July) so that it will start in 10 seconds or less, it is not required on diesel RV's.
If it is below about 30F, AND you want to leave in about 6 hours, then you should plug in the block heater for a few hours. Until the night before you are leaving, then leave it unplugged. Now if it is 0F, you might need to plug it in 2-3 days before leaving. But don't leave it plugged in for days on end.
AS for the fuel, yes as said above, use up the summer fuel before winter arrives, IF you will be running it in the winter. If you feel the need for winter fuel, and don't have it, then get the additives, and drive around the block and get it mixed in well!
Usually the engines will start at 20F without the block plugged in, and will run normally after about 30 second warm up. At 0F or -10, then the block heater is more required, however winter blend has a bunch of unleaded gas in it, so it runs fairly well - even at 0F.
Fred.