Joe417 wrote:
You may want to check it first, maybe you'll get lucky. I've seen them freeze without damaging the water heater. Some use brass elbows and valves but many I have seen have plastic.
I would guess that the PEX piping may be OK, but the plastic elbows or valves at the water heater bypass may be damaged.
This is what can happen to an Atwood water heater tank made of aluminum when not properly winterized - pic taken at my local dealer. :E

Less likely to occur with a Suburban water heater tank which is steel but in that case excess pressure is more likely to damage fittings. :(