Hector,
I went to a local Amish sawmill and bought 2 - 6 x 8 x 8 rough cut oak timbers. I cut them in half giving me 4 - 6 x 8 x 4 pieces. I then measured in about 18 inches from one end. From that 18" mark, I took a chain saw and cut the ramp angle going to nothing at the other end. I used the 8 " side for the base of the ramp thus raising the bus 6" off of the ground. Between the ramps and lifting the bus with the air system even a fat guy like me can get under it. I used this when redoing the OTR A/C system on my CC and I was able to get at everything.

This is a case where a picture is worth a thousand words. I do not know how to post a photo of the ramps on POG or I would. If a photo is needed, I can take a picture on my phone and text message or email it to you.