Bit the bullet and went with #3...
Tonight, I installed the bottom feet with an 18'-5" center.
That was the measurement for the awning poles and endcaps, plus that measurement breaks between #1 & #2 window and #5 & #6 window perfectly and doesn't interfere with my under storage bin.
In the next few days, I'll drill into my horizontal beams and install my top mounts, sealing everything with white butyl rubber.
Then I'll drill 57 holes to pop rivet the top rail holder... (Sealing with more butyl rubber) Still gotta measure where the rolled up awning will lay so it'll have a slight fall to shed water while rolled up.
To further insure the awning won't open while traveling, I'll add stainless steel lashing buckles and lash the poles to the side of the bus with webbed nylon straps with a tension buckle.
Taking it slow, because I don't like punching holes in my exterior skin in the first place
Measure 6 times, drill once