With the water heater door open you should see caulk around the water heater sheet metal flange. The exhaust vent for the flame does not go inside the RV, it is only inside the tank.
It could be that the gas line was not sealed good. Some have rubber seals some caulk or putty.
My current TC had a leak where the installer cut the hole too big and just sealed it with caulk which cracked and drew up after a year or so and started leaking. It was only about 1/8" too wide but the caulk funneled the water into it.