First you are not looking for a plug. You are looking for a valve or two I think. If the low point drains are open.
If you put water into the fresh water tank and water is coming out someplace. There are only three valves that will let water out. The fresh water drain valve and the two low point drain valves. Well there could also be a real plumbing problem also.
I had an RV that had the two low point drain valves that were visible only from the outside under the rig behind the rear wheels. Not very convenient. They are used while winterizing the plumbing.
Most often the valve to drain the fresh water tank is next to the fresh water tank or sometimes external. Where ever that may be. That water tank is drained every fall while winterizing the system.
Learn how to do it yourself is a good suggestion.