It can be quite tricky to dimension inclined walls in Revit. In the case that the wall joins are mitred it is possible to tab to the intersection of the walls, but it is a laborious process and requires one to observe closely what is selected.
It helps if one has the Thin Lines tool toggled off (the visual cues are much clearer then).
A variety of challenges and some solutions to them are contained in this Autodesk Knowledge Network page: https://knowledge.autodesk.com/search?search=aligned%20dimension&p=REVITPRODUCTS&sort=score.
Here is a reference to the workflow suggested by Autodesk, which achieves the result by cutting adjoining walls to clipboard and then pasting them back into place: https://knowledge.autodesk.com/support/revit-products/troubleshooting/caas/sfdcarticles/sfdcarticles/Aligning-dimensions-on-angled-walls.html.
I prefer the following workaround instead.
Disallow the walls join to which it is required to create the dimension. RMC on the blue dot that appears at the end of the selected wall.
The wall join tool may also be used to disallow the wall join.
After both ends have been disjoined, dimension the wall as required
Re-join the walls using the Join toggle
Or use the join tool, in which case the wall join type may be set (Squared off, Mitred or Butt)
Edit the Witness lines; use tab to select the point required if necessary.
This gives the desired result.
Subsequent to detailing the above workaround, my colleague shared his workaround: Insert a door in the wall and dimension that instead (to my mind more elegant).
Then move the witness lines to the positions on the wall required, and then delete the door.
With regards to Autodesk including the capability in future versions, please vote for this idea: http://forums.autodesk.com/t5/revit-ideas/enhancement-aligned-dimension/idi-p/6513612. I have done so this morning and added a comment illustrating the above situation. When posting new ideas remember to check whether they have not perhaps been posted already.