Had the same visibility problem in my truck with the cap on. A simple
inexpensive item I use is a type of lens that I picked up in Walmart
for around $10 I think it was. This lens is close to the size of a
sheet of regular paper and it is somewhat flexible. It sticks on the
inside of the window in my truck cap and provides me with better
visibility (I can see my small utility trailer with the lens but not
when I remove the lens). The nice thing about this lens is that it is
applied by just using water so it can be removed and reapplied any
time you want. Overall it is cheaper than a camera but it might not
provide the view a camera would.

Cheers,
Lawrence