Sorry to resurrect an old thread like this but today I ran into the same situation and now have a solve! 
First though I would like to talk about the issue. If you install MeTube and don’t configure the download directory you won’t be able to change it unless you uninstall and reinstall.
The download directory might not match other install patterns of certain other Truenas Scale apps where you might normally click on the “Add” button for “Additional Storage”, but instead you must configure your download location settings directly above that.
If you do not, and you let the default settings stay, you might find that your videos are downloading to here:
/mnt/Pool/ix-applications/releases/metube/volumes/ix_volumes/downloads$
You might also find that you cannot override even with environment variables.
Trying to override the DOWNLOAD_DIR environment variable within the app configuration gives an error:
[EFAULT] Failed to update App: WARNING: Kubernetes configuration file is group-readable. This is insecure. Location: /etc/rancher/k3s/k3s.yaml Error: UPGRADE FAILED: execution error at (metube/templates/common.yaml:11:4): Container - Environment Variable [DOWNLOAD_DIR] in [envList] tried to override the Environment Variable that is already defined in [env]
So just to reiterate the solve,
- Uninstall the MeTube app
- Reinstall it
- Edit the config
- Change your default download location directly below
MeTube Downloads Storage- This must be done on initial install because once you set this you wont be able to change it without reinstalling.