Howdy neighbor (somewhat).
My full-size conventional is 24' bumper-to-bumper....the rear tires are the widest part of the bus (excluding mirrors).
I measured the wheel spans last year (for someone here), which is why I have pics.
The board is 8' long. The front tire span (outside edge to outside edge) is 91".
In the last 2 pics (rear tires), you can barely see the ends of the 8' board on each side (pics taken looking as straight downwards as possible).
The mirrors stick out a good 6" on each side, so if that retaining wall is taller than 5 1/2' of so, you'll have issues.
Hope this helps.