We owned a Starcraft Venture pop-up -1999 new-. We had it for 6 years and when we traded for our first travel trailer, it was still in pristine condition. But, we worked very hard to make that happen. And we had all the "canvas" -more like plastic- siding replaced after the first year of ownership.
The awning flipped over the roof and punched a hole in the roof. At the tune of $4000, insurance paid to replace the entire roof. So, when sold, it was virtually still a new camper.
But I would not expect that (plastic) siding to last for 18 years. I figure, when the actual siding material was about 5 years old, it was time for a new replacement. The roof should last forever! It was aluminum, one solid sheet.
As far as leaking, it couldn't leak. The camper had no roof vent, no air conditioner, no bathroom, so really, nothing to make any holes in the roof. No holes, means no where for water to penetrate.
Lots and lots and lots of pop up camper have air conditioners mounted on their roofs. Some folks even mount bicycle racks on the roof in addition to factory mounted air conditioners. Air conditioners weigh somewhere between 70 and 100 pounds. The lifting mechanism on most pop up's will have no problem lifting an extra 100 pounds.
If shopping for a used pop-up, you need to examine the canvas siding, or plastic siding, or whatever the material is made of. This will be the most vulnerable and potentially area of greatest failure. Check the seems carefully. Make sure all the grommets and bungee cord catches are there, no rips, no tears, no shredding, no seams coming apart.
The box of the camper will most likely be just fine!
Extend both bunk ends, and look for a gap under the bunk where it sits on top of the rear and front wall. If there is a gap and you see daylight, you'll also have mosquitoes and bugs come through there.
When we were shopping for our first pop-up, I noticed this gap, almost half an inch on new Coleman pop-ups. And I think that is was biggest reason we walked away from Coleman. Also, at that time -1999- Coleman had the door that flipped down and it became the step to enter the camper. I could only see that failing under heavy weight eventually.
Look for gaps where water, light and bugs, can get through. If you find them, either figure out if you can seal them up, or have the existinging owner do that, or walk away.
I doubt you have very many leaks in a pop-up. Mold? yes. If the pop-up was ever closed up wet, you will forever be fight on-going mold. It will never completely go away. You can clean it, slow it down, but it will always eventually come back. Look for traces of mold on the canvas materials.
Like a tent, the inside of the canvas material will sweat and will get water droplets on it. This is normal to wake up in the morning, accidently touching the sides of the camper canvas, and getting splattered with a shower!