This is interesting, and completely possible, however unless there are spikes in climate or a complete bacterial infection in the water bowl, as mentioned in this example, I see very little worry in going on a trip.
3 weeks IS a long time but not when it comes to food.
If you have to take that amount of time and be away, I'd suggest dropping the temps and providing at least two water sources, maybe three. One of those sources should NOT be big enough for the snake to submerge in, only to drink from.
High temperatures are far more dangerous to snakes than lower temps at least to a certain extent. Snakes are able to adjust to cooler temps and can tolerate being slightly cooler for a certain amount of time. If you have a thermostat that is fail safe (will not fail on full power), you should have little to worry about if your vacation is during the summer months and is longer than a week.
We just came back after 9 days away and things were exactly the same as when we left. One snake shed and 2 of them dumped out.The 4th stayed in its hide and did nothing other than eliminate waste.
I shut the ambient room heater off and allowed the house to sit at 78 degrees. The AC would only come on at 79 or higher.
A quality thermostat is recommended in all instances. If you lose power for 2 weeks in the summer and your ambient temps exceed 95 degrees constantly then you may have an issue. Our summer trip temps in the home usually mimic the snake's natural environment and show little effect on the health for the snake.
Reptiles are typically great pets that need minimal supervision, if properly set up and monitored.
It is late for me tonight so excuse any grammatical errors and/or mistakes in my post.