If it's a small channel, you can just copy the channel ID. Visit their YouTube channel, look at the URL, and construct your URL like this:
https://www.youtube.com/feeds/videos.xml?channel_id=CHANNEL_ID
If it's a larger channel, then the channel ID will be replaced by custom string and you'll have to look at the page source to find the channel ID.You can also export all of your channels as OPML, which is actually documented (https://support.google.com/youtube/answer/6224202?hl=en) and upload it to your RSS reader.