About four days after I finished covering my camper shell, I asked my wife what, exactly bias tape was, and got the light-bulb moment.
Too late!
I had two seams running across the roof of the shell, lengthwise, which became really offending, between the thickness of the block filler and the lousy job of cutting I did. So I thought, why not use spackling to feather them down to the edge of the shell

That seemed to work to ease the height of the seam. If I'd have done more than one pass at it, I'm sure I could have made the seams disappear.
.02.
Mark