Travel Queens were super well made. Our first camper was a 1969 and it didn't leak a drop when it was 30 years old. Leaks are almost a death sentence for campers, so if you see any signs of leakage, know that there is likely damage to the frame in that area. For $300 though, I could stand a little frame damage here and there if it wasn't a crucial spot like a tie-down point or under the sleeping area.
If the furnace and stove work, that's excellent. If it has a refrigerator, turn it on and see if the vertical column on the back side gets hot. If it heats up, the coils look intact, and you don't smell ammonia, it likely works.
We loved our TQ.