Pete, I don't know if you have found anything to cover the ramp with yet, but I saw this and wondered why not use Rhino Liner? Or at least the Generic version of it they sell at Autozone for less than 50 bucks or so.
I would think It would hold up to the abuse pretty well, and it is also made to hold up to the elements. It would give you the traction you wanted at a fraction of the cost of grip tape. Just a thought.
Totally built from the ground up; weell done. I'm doing something similar; on a slightly different tangent. Bike hauler; CT conversion; the new 6X10 trailer cost 2,800 + 1,500 in materials to date ... I used 4 ply 3/8" finished one side exterior fir to cover the interior; you may want to consider to add rigitity and finish over the insulation.