Select the wall you want to slope then click green wall icon second from left
in there you can change the height of one side or the other - if you change the center height you will make the whole wall the same height.
your wall should start with the tallest height then set one end to the shorter height and it will slope.
now with all that said if you need to show a peak in the center of a wall the only way i have found is to split the wall in half - then adjust one side - and adjust the other.
you would need a picture of what you want it to look like or just a color to put on the cube but you can make the cube the size you need and maybe 1" thick then align it in place