If you are essentially 'camping' then there are off the shelf options Camping Hot Water Systems.
My recommendation would be the Joolca Hottap Outing https://www.joolca.co.nz/hottapouting I haven't personally used one but it is essentially the same as how I have mine set up as a self contained unit.
Stay away from the Coleman hot water on demand unit - my brother had one and it split the internals after a couple of months. POS.
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