We planted maple trees in the yard but now realize that they are close to the edge of the slope downgrade to the road. Should we build a wall foundation around the tree to raise up the ground level around the tree where the slope is? Will the tree roots be strong enough to hold the tree upright near the slope?

I am not a tree expert but worked for a state highway agency that planted trees on slopes all the time. I believe that the primary concern is that the water runs down the slope and does not reach the roots, but there are ways of preventing that. This link has some info on that sunject on page 633-7:
http://cmg.colostate.edu/gardennotes/633.pdf