Hi Guy, assuming you have the circulation pump etc. on your Aux heater it is fine, that's what people use these days as a modern alternative to the plug-in electric engine heaters in Scandinavia. Things have moved on in 30 years
Just bear in mind that it will drain the battery, in case you do short trips while in Tromso.
Also make sure your coolant and windscreen washer mixture can handle the sub-zero temperatures.
What an epic trip, your trip to Vienna "en-route" is only 800 extra miles, so it will be a cool 3000 miles one-way (assuming shortest route)
