If you are going to be on the freeway, get new tires. Also, if they are those small tires, like what I see on some pop up's, replace them. Think about how fast they are rotating at 65+mph. If it were a small "garbage trailer" that sees the road once a year, wouldn't go faster then 50mph...sure, tube it.
I purchased a utility trailer at an auction and it was as you said, great tread, but side walls were cracked (bad). I ended up getting used tires (actually older then what I had), but were better condition. I don't plan to use this utility trailer very far and I trust the tires I got (just an odd ball size....14" trailer rims).
A lot of my vehicles have tires that are older then 6 years old. I don't drive that much, and since I have a few vehicles, they don't get many miles on them. The last set I bought "new" for my F150 was just 2 years ago to replace a set that I bought back near 15 years ago. Tread was getting low (plus noticed a bulge).
That trailer aid seems nifty. I should get one for the equipment trailer....or make something out of some wood. Says online 15,000lb rating. Unfortunately, I"d end up using it for my tri-axle equipment trailer w/ 14.5" Daytons...