I suspect your mechanic was just being honest with you, that the value of the trailer was less than the cost of replacing the roof. That sounds pretty honest to me. So if you can afford the new roof, maybe you can afford a used trailer from the same age. Or maybe you like the trailer and don't want to trade no matter the value.
As far as a soft roof, I think they all use plywood that is barely strong enough to support a person who is careful. In other words, none are really very strong up there. That keeps weight and cost down. If the rubber is in good shape, and you don't break through when walking on it, why not just leave it alone?
If you're handy, you could do the roof yourself. Just sheath it a little heavier and install new rubber. I would probably add sheathing anytime I was replacing the rubber, because like I said, they are all pretty thin.
Most important is whether it has any water damage. And you probably don't know.
2006 F250 Diesel
2011 Keystone Cougar 278RKSWE Fiver