Atwood rv water heater model G6A-8E heats water to
Normal causes for that would be by-pass valve not in proper position, or defective, allowing cold to mix with hot, or thermostat not touching up against tank properly, or defective. Do you know where that thermostat is?
Look where the black foam cover is, with white letters, below pressure relief valve. One is lettered ECO, other THERM. If you note where the wires go, so as not to mix them up , and pull them off, then peel back the foam cover, you'll see 2 little round discs, that are held against the tank by a small spring. They need to be checked that they are in there properly and are up against the aluminum tank.