I have a Jayco 2007 165 too and there was no gauge for the camper. We did one year without a gauge until the gray water overflow in the shower. I can tell you something, it stink.
I almost think that it is worst than the black water because there is nothing added to it for helping with odors. I installed a Seelevel system. It has a gauge for gray water, black water, fresh water, battery and LP(which I didn't connect the LP). If you you would like more info just let me know and I could post some pictures for you. I love the product and there is no more guesses and overflow. It is installed on the outside of your tanks and as a better reading than what you would get on your TT from the manufacture.