My experience is the gray and fresh tank indicators work most of the time, but not very well. The black tank indicators work for the first 2 days and that's it. If your toilet has a straight pipe down to the black tank (hopefully) you can turn off the water supply, open the flapper, and look down with a flashlight for a very accurate indicator of black level. Never, ever, use a flashlight that will fit down through the pipe--only use a big rectangular one that will not fit--no exceptions! As said above, experience will quickly teach how long one can go.
Off subject but since OP is newbie (we all were once) I'll offer two other pieces of advice. After checking the black level, be sure to get a little water on the toilet flapper valve to keep the seal moist--you always want some liquid on it. Dried out seals leak stinky gas. Also, do not let your black tank continuously drain like we did our first week out. Let it build up as much as your schedule allows so solids are immersed in fluids and stay soft. Always whoosh out when draining a black tank.
Have fun camping!
