I installed a single sheet on the floor of my camper a while ago, and then installed 12"x12" self-adhesive squares on the roof this past weekend. They were both a bit tedious for different reasons. One large sheet, or even two medium ones are difficult to align properly and it’s difficult to apply the adhesive and then position it. With the smaller square foot pieces, they are easier to align, but you have to do it repeatedly and I ended up with small slivers between the tiles that allowed sunlight to shine through (even when muted by the honeycomb composite roof). I also found that in areas where I got two pieces too close together, one would invariably be beneath the other, causing the adhesive on at the edge to miss the roof and allow it to droop slightly. I went back 24 hours later and found many seams were buckling. The tiles also came in slightly different colors, making it look worse overall. Amateur hour over here for sure 
TL;DR - I prefer how the single sheet looks, but the smaller, self-adhesive tiles were much easier to install.
I haven’t noticed condensation in the carpet headliner in the locations/situations I had noticeable condensation develop before (near the beach, lakes, rivers, and rain) . The exposed aluminum around the carpet still collects condensation, but that is an easy wipe with a cloth.
Absolutely, you could cut it in two pieces. But again, the hardest part was cutting the carpet to the exact size of the roof panel (you can see I didn’t get it exact, there is about 1/8 of an ich on the left side you can see in the picture). I took my time installing it. I sprayed the 3M Headliner Adhesive in two foot sections on both the carpet and roof and let it cure before pressing it together (I did a double coat at the start and finish). To make it easy I started at the front of the wedge and worked my way back/higher. I believe it took about 2.5 hours to cut and hang the headliner.
I had originally thought easy it would be to use self stick carpet squares, but another GFC owner reported his attempt with carpet squares left a sticky mess after they all fell off one hot day. I hope you have better luck.
Thanks. I’m assuming you masked up some plastic sheeting or something on the sides of the tent and non-removable bed piece for overspray?
This looks awesome!!!
Now that you mention it, I used some Christmas wrapping paper and masking tape to protect the tent material.
I think I used the Elevations Carpet in Sky Grey from Home Depot, it was on a large roll and had to be cut by a Home Depot employee. It has no backing like traditional home carpet so is light and easy to work with. I bought 2 linear yards. Looks like it went up in price a little, I remember it being .37 a square foot.
When looking for 3M Headliner Adhesive spray I found O’Reilly’s to have the best price and in stock locally. I had to call around to make sure it is in stock, the website showed in stock by the first store I went to didn’t have the 3M brand. If I remember correctly the 3M Headliner Spray had a higher temperature rating than their other adhesives. I used about one and quarter cans.
I can’t find the original “Headliner in tent” thread here on the GFC Forum, but that thread is where I got the guidance to use the Home Depot carpet and the information to the proper adhesives.
I was one of the second folks (I believe) on here to do the carpet headliner after Phil. The headliner did get wet, but it prevented the water from running down to your feet and getting them wet. I always aired out the tent after camping (even in fair weather) so the carpet always dried. There was never a worry that it would get moldy or mildewy since it dried out fairly quickly anyways.
I used this stuff: Nonslip Foam Flooring Gray 27" x 94" (Adhesive Backed) - Sailrite
It’s self adhesive, USA made with a quality 3m adhesive. The foam ought to provide a little bit more r-value/insulation. And if you ever need to remove it the adhesive will be easier to remove than spray adhesives.
Two sheets were almost a perfect fit for full coverage on an XL camper.
This stuff looks similar, is slightly less axpensive (not 3m, though) and is probably made in china: https://www.amazon.com/dp/B0CSM4ZM4D/?th=1&psc=1