Ramps, Spitter Valves

[cdat]

I discovered that two of the spitter valves on my air system tanks are leaking and need to be replaced. I spoke with Newell, their recommendation was to replace them with standard petcocks, should be easy enough.
Since I will have to drain the air from the tanks I will need to put the coach up on ramps in order to have enough room to get underneath it and work. My plan is to use some fence posts, (similar to railroad ties), for the ramps.
I know I need to put the ramps under both duals, but do I need to support the tag axles also?
The reason I'm choosing fence posts are, cheaper, easier to get, and for the most part seem to be in better shape, harder to find railroad ties that aren't cracked. Size is about the same, 6" x 8".

Any thoughts?

John