Heres another theory....the forklift was under the pin while they raised the lift, but it didn't go up in sync with the lift and in the process they pinched the sewer line, causing a break.
Clearly they did the damage if it didn't leak going in, but did coming out.
Something else....I'd climb up top and see if you see anything up there. Its awfully close to the top of the garage door in that one pic and if they creased the top of your trailer, or broke something loose on the roof, you want to find it before you get lots of water damage from up there.