I’ve had a touring caravan out in the wilds of Wales for 15+ years, no electric on the site, yet I can comfortably stay there with everything I need.
I bought 2 second hand leisure batteries, £10 each, a solar charging panel, £30 & an 800w inverter.
Hooked the 2 batteries up together with the solar panel charging them, inverter hanging off the batteries which then feeds electric to the caravan, jobsagoodun, I can comfortably run all the lights I need & a couple of hours of TV a night for over 2 weeks doing it this way.
as an added bonus the caravan uses it’s built in trickle charger to charge the batteries when it’s hooked up to the inverter.
So I’d say all you’d need is 1 leisure battery & a solar charger, then hook up a few lights to it & you’re away, or for much cheapness, just take the battery into the house every few weeks to give it a charge, or as I’ve had to do once or twice in the past hook it up to the car via one of those cigarette lighter to battery charging leads & it’ll charge as you’re running about doing your everyday things.
Water butt should be ok this time of year, but it’ll get bad quickly if once the weather warms up, then there’s the worry of kids tipping it over for a laugh, unless it was kept inside the garage & filled up daily as you mentioned.