What is the maximum potential energy on the inclined plane?

The potential energy of any body of mass M kg raised by H meters above the earth's surface is M x H x G (G = 9.81 meters/sec/sec). It doesn't matter whether it moves up a plane or vertically, it is only the vertical rise that determines the potential energy. Of course on a plane the movement may create friction which means you have to put extra energy in, but this extra energy does not count towards the potential, it is probably lost in creating heat.