I over filled but it all came out the vent on the side of the mh.
Those tank expand. If it had been left overfilling longer it might have ruptured.
After I cut the water off it still spouted out the overflow with the drain opened too for several minutes.
Thank goodness all is well, I think.
In stead of putting the gauges in the wet bay they put them in the overhead inside the mh. So you have to rush from wet bay to inside the mh and back to not overfill, including the black and gray tanks when you fill to flush them.
It seems a simple job to put a second set of gauges in the wet bay if I can find someone who knows his way around a mh.