My opinion is, every used unit will have a certain amount of water intrusion damage, depending on if the owner stayed after the seals or not, however, slight damage is nothing compared to advanced rot, mold and fungus growing between the walls and outer skin.
Not all that hard to tell either.
Just look carefully where the walls join the floor, inside cabinets and especially inside cabinets and under the bedding in the cabover. If it's going to leak, it will leak first in the cabover area, thats where the rain pounds on it when it's going down the road and if they leak, thats the first spot.
Another real good way to determine if there is water intrusion and rot is use an infrared hand held thermometer and check the walls in the cabover and near the floor. Rotting wood and mold growth causes heating and a handheld digital thermometer will pick that right up.
If it has water intrusion, the next thing you need to determine is do you want to fix it or pass on the unit. Me, I'd pass. Refurbing a TC is not an easy job.