Tape is good for sealing but not much good for support.
One way to support the plastic covering is to place some 1" X 2" wood strips from side to side where the plastic joint is located. Cut them long enough to reach from the vertical part of the frame on each side and sit on the top of the bottom frame flange.
Then secure the plastic sheeting to the wood strips using 3/4" sheet metal (or drywall) screws and fender washers (about 1" diameter with small "screw sized" holes in them. Center the wood on the seam and put screws in in pairs, one on one side, the other on the opposite.
If not enough room to maneuver the wood strips so they sit on top of the bottom frame flange just cut long enough to fit between the flanges themselves and lay a foot long piece of the same material on top of the flange on each side and secure with longer screws. The top strip will hold it all up.
When complete, THEN seal with tape. To make tape stick better wipe plastic down with Alcohol, Laquer Thinner, or Acetone to take any dirt and oils off the plastic. Tape will then stick a LOT better.
"A wise man can change his mind. A fool never will."
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2018 Flagstaff Micro Lite 25BDS
2004 Nissan Titan