If i understand correctly, your ducting is only used internally.
That being the case, insulating the ducting is a waste of time, any heat losses will be lost to the inside of the caravan.
I only use the heating on electric, and after several attempts to balance the air distribution, i gave it up as a bad job.
What we do is only use the fire on convection, never use the fan.
But in addition use a fan heater on 1kw, on its stat, we find this very efficient.
As mr watson states you need to use 3kw gas to get the best heat, apparently some fires allow both fuels. So you have 5kw on hand.