15,000 lb tow rating - fulltiming
Look for a 5th wheel around 12,500 - 13,000 fully loaded weight. You will need the extra cushion for decent performance, especially if you every go near a mountain.
Full-time, part-time, weekender - the floor plan is critical. Find a floor plan the WIFE likes. That may be the most important part.
Practice how you get past each other in the bedroom, the hallway, the kitchen. Etc. Look closely at clothing storage and kitchen storage. The wife will use more clothing storage than you. And the kitchen is her workshop. Make sure she is happy.
Once you get an idea about which floor plan is best - search on the internet - identify the brands with that floor plan - then go looking.
Posting the "Is _____ a good quality trailer?" will get you one or two comments from people who actually know something about the brand. And 50 posts about how you should buy X trailer, which is usually out of your price range.
IMHO the most important thing is the dealer. Ask them for references from people who have purchased from them. When you see some rig with the dealer's sticker on it - ask the fellow about the dealer and their service. All rigs need some service at some point.
5th wheels usually have a very good outside storage. Don't plan to fill it up. You can easily go overweight with "I might need" stuff.
Other than extensions for power, water and sewer - I go through everything in my rig twice a year. If I haven't used it in the last six months - it goes away.
You may need extensions only once a year, but when you need them, you need them.
Crawl through the rig on the dealer lot. Crawl underneath the rig. Salesmen hate me because I ask questions they often don't know the answer to - and if they won't get a service man who does, I walk away.
Be very, very picky about everything!!!