Level

It may be common to mount that large size on the front of the pin box on fifth wheels. I have installed the smaller ones that come with adhesive backing. I also put caulk along the side edges to hold them in place because the adhesive backing seemed to shift.

If I were going to screw them on I'd predrill and use small stainless screws with caulk.

Drilling pilot holes for the screws will prevent the fiberglass from cracking.

I screwed two levels to the front and sides of my trailer using stainless screws. Drill a pilot hole to fit the body of the screw, put a small bit of caulk in and around the hole, press the level into place, and carefully screw in the screws.

Most levels have a self stick backing on them which I use but use the SS screws also. I have never had a problem with them coming loose, shifting, or the fiberglass cracking. Just get the pilot hole the correct size and do not over tighten the screws. :) Also, make SURE the trailer is level before installing the level indicators. I use a carpenters level on the floor but many use a smaller one inside the fridge to make sure the camper/fridge is level.
Barney

Mine are smaller...I don't mind getting out of the truck to check them ....have to get out sometime anyway.
I used 3M Extreme 2 sided tape.

The levels that were on my trailer when I bought it were attached with only the adhesive. (No screws). I lost one to highway winds on my most recent sojourn.

The one I bought to replace it also came with adhesive. I too am hesitant about putting holes in my fiberglass, although I know that is an unreasonable fear. The problem with the adhesive method is that it is an adhesive FOAM, and it doesn't give you a rock-solid attachment. In other words, in spite of my very-careful installation on a perfectly level trailer, the level can rotate small amounts. To be totally reliable, it has to be calibrated (leveled) before you can rely on it. Each time.

Did you know that your iPhone has an app that is a level? Its in the "extras" app. It's "behind" the compass. Very useful to know.

-Speak

Mount it with some 3M Outdoor Double Sided Tape. The stuff is really tough stuff and meant for permanent installations. I would say that it's equivalent to the adhesives used for vehicle decals. If you line the back of the level with that, then it's not going anywhere. It's better than drilling unnessary holes in your trailer. Every hole in your trailer is a potential leak point. The fewer the better.

As others have said, drill a pilot hole first. I did caulk around the level and also used butyl tape to seal behind it.
Your camper has a bajillion holes in it already. A couple more really is not a big deal.

I’m in the no drill crowd, use 3M VHB tape.

proxim2020 wrote:
Mount it with some 3M Outdoor Double Sided Tape. The stuff is really tough stuff and meant for permanent installations. I would say that it's equivalent to the adhesives used for vehicle decals. If you line the back of the level with that, then it's not going anywhere. It's better than drilling unnessary holes in your trailer. Every hole in your trailer is a potential leak point. The fewer the better.

x2 did that 8 years ago and it's still attached.

I use the sticky tape too. Holes not necessary shouldn't be drilled.