Since you're in the market for a used pop-up the condition of the unit should be first & foremost. I have owned 6 over a period of 2 years, all purchased used. As far as quality & construction Coleman & Starcraft are the very best. You may want to start your search by narrowing it down to these two manufacturers & going from there. Having said that, condition of a used one is still most important.
Mold or mildew stains may be a common problem in high humidity climates, but most coaches should not have an obvious odor or smell to them as you indicated. A few stains in the covering materials won't cause an overwhelming odor when you walk into it. It takes a little effort, but my recommendation is to crawl under the coach & inspect it front to back, looking for any obvious water damaged & or soft spots in the flooring. Also, look at the siding of the trailer for bulges or abnormalities, this can indicate rotten wood behind the siding. Stains on the ceiling are always obvious but may not be a serious indicator of problems, the coach may of had a minor leak at one time that has since been repaired. Another thing to do is open the cabinet doors and inspect the flooring from inside the coach, but don't forget to inspect it from below. ( most important ). PS: the 6 units I purchased & used was over a period of 2 years doing minor repairs and reselling, making enough net profit to buy a 22' hard side & paying cash.
I'm an expert in only one field....I believe it's somewhere in Kansas.
2000 / 22' SKYLINE NOMAD LITE
1998 DODGE DAKOTA / 5.2L= 8mpg.
2006 POLARIS ATV
1500/1200 Watt Champion generator
Yada Wireless Back Up Camera
1998 Dyna Wide Glide
USMC 68-74