I did the same thing early in this camping season. After blowing lines for winterizing (I usually run the pink stuff), I forgot the by-pass was on. When filling up with water, water came out the faucet just fine! It sputtered and I though all the air was out. Nope! Long story short...
I fired it up on electric first. Burned up the electric heating element.
Still didn't realize there was no water in the tank and flipped on gas.
It fired, I though everything was ok. Later tried the water ... cold (still on bypass)... Then it hit me. OH ****!
Well, the gas would not fire and electric was dead..
Long story short, took it to my dealer for repair. Replaced the electric heating element and the thermostat! yup, I destroyed the thermostat!
Got it fixed and everything has been fine since.