Strangest thing, I just read a thread about your truck on an EV forum yesterday. Now I see your post here.
I think you're on the right path. You're proposed shape reminds me of the Don-Bur trailers in the UK.
https://donbur.co.uk/ They claim less drag even with more frontal area.
That is to say. I
think you can lower your drag even though you add frontal area.