Petra,
Before spending big $ on generators and high power heaters, I'd recommend you try low power USB style heater wrapped around the lens hood but with some closed cell foam as insulation wrapped around the outside of that. eg. yoga mat material. These consume only about 2-3 watts each. 5A @ 12V is 60W and sounds like way too much heat to me. It obviously depends upon your local dew/fog conditions.
Try at first, buying just one of these and mount on your biggest lens with some insulation around the outside:-
https://tinyurl.com/mtuj84sm.
It will probably work fine. If it doesn't do the job on your big lens, no money wasted, because it definitely will take care of your finder and/or a small guide scope.
You can run 3 of these plus a scope for a whole night off a small car battery. Check the power consumption of your mounting. My mount tracks at 1A, slews at 3A. I bought a 40 AH battery in 2015. It's still going strong 7 years and hundreds of recharges later. My three USB heaters draw 2W each, 0.5A total for three heaters. When photographing, typically I am not repeatedly slewing. So the total current draw is 1A (mount) + 0.5A (3xheaters) = 1.5A. Over 8hrs, that's about 12AH which the 40AH battery handles easily. I am not powering a cooled camera which can easily add 3A to the consumption.
This Century battery is 40AH, weighs 10kg, has a carry handle, and costs $170:-
https://www.supercheapauto.com.au/p/...start=68&sz=60
If you are a power lifter, then consider a bigger battery. I prefer the smaller lighter batteries even though they only get me through one night before needing recharging. You will also need multiple 12V/USB supplies, one per heater or a USB hub with sufficient power for all the heaters, plus enough accessory cigarette sockets to power everything. I built my own power hub (photo) The USB PCB's cost $1 ea, the box, and cigarette lighter plug and the RCA jack panel about $15 in total. The cable was recycled from a household fan that was dead.
You will also need to buy a car battery charger.
In addition to 3 x USB, my box includes the 4 x RCA panel which is supplied by a separate buck/boost DC-DC converter. So I can dial in any voltage from 1.5V-35V to these jacks. They are currently set at a low voltage 3.5v for some home made heated camera lens hoods I have that are fitted with RCA connectors.
Joe