I have a gauge, it's called my wife's nose :-) We pump weekly through the hot summer months. My wife does not allow paper down the hole (unless it is an emergency with marine/RV type paper). I had to help a dockmate 2 weeks ago extract a feminine hygiene product out of his 2700scr. Not fun!
Tank gauges were not even optional equipment. The best location for the gauge itself is where it can be conveniently viewed. I would not want to break down a table or move coolers to physically view the tank itself. Many folks put them in the head or the salon. HEad, so they know what is going on at the time they are flushing. I prefer it in the salon. I don't want to have to go into the head to determine if I need a pump-out.