They make tanks, with gauges, in both 20lb and 30lb sizes. Their gauges are not very accurate. They make stick on strip gauge, that's as accurate or $$$ gauges, that many use.
Run the left one empty and refill once it changes over. Left one, is normally easier to access and change out, if you're a lefty, you may find right one easier.
It's hard enough to get tanks filled correctly, nowadays. I'd just take it off, for refilling. Many change out 30lb for 20lb tanks, because of weight. Unless you're using generator or furnace a lot, 30lb tank last a long time. I carry 20lb tank, for fish fryer and I can exchange that if needed.