From Corona I would take the 10 to 111 to the 86 to Westmorland. At the stop sign past the Shell station take a right, which is HWY S30. ( At the Shell station you can fill your water tanks, then dump your waste on your way back from G.W.) This will take you to the 8 FWY, takes about 20min. from the stop sign and is a very nice two lane road with very little traffic other than a few truckers.
This way you bypass going through Brawley traffic and all the traffic signals and if you need to stop for food or gas you always can in El Centro. This route is all flat with no grades or narrow canyon roads to deal with. Just set your cruise and enjoy the ride, only problem to deal with is the wind through Palm Springs area.