Way over thinking this and adding considerable work.
If this is to be a "mobile" system (campground to another campground) you will have to fill and drain a lot of water then figure out how to haul a drum around..
Lots of "overhead" trying to heat all that water above ambient air temp, will take considerable amount of time to see any benefit, especially if you are filling from empty with very cold water.
Drum of water will cool down considerably when the sun goes down, especially during winter nights, even in the south..
What you are trying to do is called "batch" solar heating, which can work but typically it is done using a solar panel and a well insulated tank.. in the daytime the water is circulated through the solar panel, heating the water in the tank. At night, the water is not allowed to flow through the solar panel (flow through the panel would release the stored heat back into the outside)
I suspect, you can do the exact same thing with just a length of black pipe laying on the ground or even laid out in a couple of rows on the roof of your RV.. Some folks have just coiled up black pipe into neat coils with pretty good success.. Depending on the size of coil(s) you make it should be easy to store when traveling!
Simple, yet very effective without all the hassles..
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