I would approach that trailer (purchasing) very carefully. That roof was designed to hold up that A/C unit, and, having owned an '08 210WBS, the roof is very sound and capable of being walked on. If the roof "couldn't support the A/C unit...", then the roof had probably leaked around the A/C unit and has caused some rot of the rafters...a leaking roof, obviously, is not a good thing.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)