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We don’t have one, I can’t advise on the 12v / battery side which might affect your choice, but others will do that. Meanwhile, try https://www.bestproductsreviews.co.uk/best-12v-electric-blanket-for-camping
Can't recommend any particular one, as no personal experience. A quick Google suggest a plethora of obscure (Chinese?) brands, but no familiar quality/reputable/know names to me!
In some ways, personal heating over 'space' heating is wise if using 12v as the power source, but 12v and ANY heating device are pretty poor bed fellows due to the heavy demands on the battery and need to recharge frequently. From what I can see, most 12v electric blankets/throws running off a 12v vehicle supply are rated around 50W (some are much higher!), that's 4.2 amps drawn, which is pretty high for a continuous draw. A 100Ah leisure battery (only half of the rated capacity is usable on a lead acid battery before it reaches damaging levels of discharge, so consider it a 50Ah resource) would be run flat in around 10 hours, that's assuming the battery was in perfect as new condition, but they do decline with age/use, a still generally considered 'good' battery may only give 7 or 8 hours use. You would need to recharge the battery ASAP to prevent permanent over discharge damage! You would also need to ideally monitor the battery state of charge to ensure it is not over discharged.
The situation improves slightly, potentially at least, if the blanket has some thermostatic control and/or lower power settings that would not deplete the battery so fast.
There are some blankets that have their own battery pack, but you, much as with your leisure battery, need a means of recharging them after each use.
Various Jackery type 'power packs' could be used instead of the leisure battery as the power source, most have Lithium batteries which have a high power density for size/weight compared to a lead acid leisure battery, so may power your blanket for a few days use, but ultimately will require recharging. Sadly they come at significant cost!
If off grid, solar charging could be a solution to battery charging, but if you're needing heating, then I'd guess it'd be out of peak summer months, and solar panel efficiency plumets at those times due to limited sun and low sun angles, so far higher power rated panels are needed to maintain decent output, again at some significant cost.
Not really a definitive answer, as it's a problem with multiple variables, a bit 'how long is a piece of string' situation. Hope the info is an insight, if not an absolute answer.
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