Every single thing.
Look on line for a Travel Trailer PDI list you'll find plenty. Plan on it taking a long time and ignore the dealer that want's you to just take the TT and leave him to go back to playing video games on his computer.
Also, do the PDI BEFORE you close (and pay) for the coach. Don't buy it if stuff doesn't work or is missing. They will all promise to fix it as soon as you sign on the dotted line. They'll say, "Hey, we're friends and will take good care of you." But the minute you pay and leave with the trails they'll forget you ever existed.
PS. Oh, PDI stands for "Pre-Delivery Inspection."
2017 Winnebago Adventurer 37F
2016 Lincoln MKX Toad