Ken pretty much covers it, test every single item for functionality.
Because it can't be mentioned enough, I will stress it again: Bring a ladder and triple check the roof and, most importantly, roof seams for cracking or if Dicor has been used as necessary to treat any cracks. That will give you the best assurance the trailer is not leaking, which is the biggest problem you don't want to run into. Anything else that breaks in the future can generally be replaced/repaired fairly easily, as opposed to repairing a massive water leak.
Also: Check the tires for odd wear patterns. That will alert you if an axle may be bad.
2014 Bullet Premier 29bh in Charocal
2019 Ram 2500 HD 4x4, CC, 6.4L
2011 Passport 2510RB (Sold)