Water – Shut off valves to tankless hot water heater won’t turn

I don't think that you need to turn off the gas to the hot water heater. I believe that it is likely acceptable to leave the appliance on the 'pilot' setting.

I would obtain the manual for this device (try searching online, you will likely find it that way) or call the support number for the manufacturer.

However, what is acceptable practice is not necessarily best practice.

Personally, I would be hesitant to leave a gas appliance unattended for more than a day or two without physically disconnecting the gas (usually a 1/4 turn valve on the feed line). However, I feel comfortable lighting the pilot light myself.

Perhaps your should check with the local utility that provides your gas and see if they offer a service to visit your home and relight the pilot light upon your return? Mine does and it is free. My wife knows to contact them in the event I am unavailable because she is not comfortable at all in doing this herself.

Very odd, but here's a thought on how to troubleshoot. Shut off water pressure into your water heater. Test each one of your hot water taps. If any of them work, something is mixing cold water into your hot water pipes. Some one-handle faucet and shower fixtures can do this if their internal seals are worn out. Find it and fix it. If they all only work when water pressure is supplied to your water heater, then the problem is likely the water heater itself.

Are the other units on the same piping? If so, the mix may be caused by a recent change in their plumbing. Maybe you can engage them in the troubleshooting.