One of my To-Do list items was a cab roof deflector.
I delayed installation my first 11 months of full-time RV life. I didn't trust the strength of
my truck cap. The sawed off aluminum topper covers the front third of my 6.5' truck bed. (The nose of my 5th wheel trailer covers the back third. The middle third gap is mostly filled by a generator mounted on the nose of the trailer.)
Cap strength was a concern because of how it was shortened. After peeling off the skin the under frame was cut down. The frame was originally all welded. I can't weld aluminum so I used pop rivets, screws and bolts. It holds itself together no problem. I'd trust that in a hurricane. What I don't trust at highway speed is additional stress from a deflector. Not until I get around to reinforcing it better.
The original plan was to use the deflector as intended. Eventually replace the fiberglass wing with a solar panel. Last month I (finally) added solar to my rig.
I scavenged the deflector for parts to mount the panel. The tilt mechanism was my objective. All this I trust inclined for stationary use but for travel I keep it folded down flat (horizontal). Highway wind loads are reasonably low that way.
What remains to do aerowise is reinforce everything to handle road winds. Whenever I get around to that I'll be in a position to experiment with different angles. It's worth doing but lower than other priorities for me now.
Even then I'm not expecting huge gains. My trailer already has generous radii (circa 12") on the top and sides. By rights bow splash should not be an issue (per work by others). A deflector probably won't do a lot of good if onset flow is already attached. Still, it's something I'll be able to do (eventually) so it remains on my someday-maybe list.
By contrast the friend's Apex doesn't have such generous edge radii going for it. If it has considerable bow splash currently then a deflector seems a prudent addition to the rig.
Here's a link to my blog post with more pix and context on the not necessarily aero facets of my deflector related project.
Solar Project – Part 5 | Recreation Engineer