2017 Montana 3100RL 5th wheel GelCoat

Looks like it was shot with a bb or pellet gun.

Or perhaps a grass mower throwing gravel

Very strange! If you blow up the pic you can see many more holes forming. Looks like something got on it that's attacking it - maybe you drove through something? If so, ins. should fix.
If you could get a new piece of Filon, you could take the door apart and replace the skin.

Have the same on a 2001 Okanagan 5th wheel, and would swear it's from a pellet gun ect. Can't prove it but have the same on both side's, small but annoying.

JimK-NY wrote:
Looks like it was shot with a bb or pellet gun.

I would think that a gun would damage a MUCH wider area and less dense. This damage is the the bottom 2-3 inches of this one door. No other place

Caused by water getting in to the fiberglass panel core and freezing during the winter. will appear as little blisters or holes as water expands during freezing and pushes the top layer off. Likely water is getting in either along the top seam of the door or around any attached handle or lock.
To fix, eliminate source of water intrusion, use fiber glass filler like Bondo to fill holes, sand to smooth out repair and spray paint door matching color.

IB853347201 wrote:
Caused by water getting in to the fiberglass panel core and freezing during the winter. will appear as little blisters or holes as water expands during freezing and pushes the top layer off. Likely water is getting in either along the top seam of the door or around any attached handle or lock.

I think this is the best answer. I will recheck for water.

Gel coat blisters.
Since it’s just the door (very lucky) it’s an easy fix.

I agree that it's water. From your picture it looks like that rounded corner has had water it/around it for a while. Over time that will migrate through out the door. The path of least resistance.
Just my opinion and experience as I had this issue on my battery compartment door.Thank fully caught it soon and it wasn't as bad as your. Sealed it and with some proper body filler, sanding and white spray paint and it looks like factory.

Yeah since it’s a little door, you could seal it up, prep it, bondo and rattle can it for under $50 and have a 50% chance it will hold. Or put a new panel in the door for not a bunch more $ if at all (doesn’t have to be filon, many options that will look good) and have a 100% chance it won’t pop again.