Sounds really dumb to me. As you mentioned, it adds weight
. It also costs money. It makes doing any repairs on the outside nearly impossible. Have you done any fiberglass repair work? Step 1 is to grind the fiberglass down, taper both sides. Imagine if someone put bed liner on top of the fiberglass. I would not be happy. Sealing around hatches, or adding hatches would also be MUCH more difficult.
Have you ever tried to clean bed liner? Nearly impossible, always looks dirty, and it traps dirt. And if you go with black, its going to be a flat black camper, absorbs heat.
Lastly, with some cleaning, fiberglass bodies on these trailers last nearly forever. Notice the number of good looking, 45 to 50 year old molded FG trailers out there? Have you ever seen 50 year old bed liner?
Now on the lower front of a trailer, it can make some sense as a rock/chip guard.
IMHO, a freshly cleaned and polished 45 year old molded FG trailer looks so good. Pictures are before I repaired belly band and door sag.
If I was going to drop $$ on a vintage trailer, I'd focus on the frame first. Then I would upgrade electrical
. And I would do all the maintenance stuff.