My car has a reliable gauge that I can see all the time. There's even a thing that tells me how many miles I have left (mas o menos).
My RV has a series of disappointing gauges that have let me down over the years. I'm pretty good about refilling before trips when I think I need it, but having a reserve tank is a great failsafe for me.
Regardless, I'd never put myself in the situation where I might carelessly run both tanks dry. I just don't trust myself or the rest of the family. I've caught my wife forgetting to turn off the oven too many times.