Since you are having fun with this, I will keep throwing in additional wild conjecture ideas to kick around.
Make the inner tent using electric blankets.
Your battery is nominal 225 amp-hr x 12 volt lifeline. If we dedicate 50% of its usable capacity to heating the tent, then:
(225 amp-hrs) x (50% usable) x (50% dedicated to tent heating) = ~ 50 amp-hrs.
(50 amp-hrs) x (12 volts) ~ 600 watt-hrs.
(600 watt-hrs) / (12 hrs) ~ 50 watts of heating
Not sure what an electric blanket uses.
12 hrs gives it a chance to pre-warm the inner air a bit before going to bed.
Make the inner tent using electric blankets.
Your battery is nominal 225 amp-hr x 12 volt lifeline. If we dedicate 50% of its usable capacity to heating the tent, then:
(225 amp-hrs) x (50% usable) x (50% dedicated to tent heating) = ~ 50 amp-hrs.
(50 amp-hrs) x (12 volts) ~ 600 watt-hrs.
(600 watt-hrs) / (12 hrs) ~ 50 watts of heating
Not sure what an electric blanket uses.
12 hrs gives it a chance to pre-warm the inner air a bit before going to bed.