That's an
unbelievable amount of daily evaporation. What temperature and has it been verified with another thermometer?
Is it in a hot room so that the tank has to be cooled with a fan blowing on the water?
Here is the easy method of auto top-off:
1. 5 gal bucket of water elevated just above the tank,
2. airline tubing with stiff plastic tube reaching to the bottom or small rock rubber banded to end of tubing,
3. plastic air valve or loose knot at the other end adjusted to drip water into the tank, at a rate that is equal or slightly less than evaporation rate,
4. start the siphon and run it several days before leaving town, to check and adjust the drip rate.
I have a 50' roll if you need airline tubing and I have some stiff tubing as well.
I also have several float valves, but honestly, with those there is the trouble of mounting them on something that sits in the tank and then the risk of failure when, not if, the mount slips. A simple, uncomplicated set-up is less prone to failure.
Aloha