doonboggle

Just crawled under my unit since getting it several days ago. Wanted to familiarize myself with it some more. Noticed 3 drain petcocks and wondering what they service.
Two of them are located at the area where the gray/black outlet is located. The other one is up forward more under the area of the fresh water tank and hw heater.

I presume the last one referred to is to drain the fresh water tank since it is in the same area as the hw heater. Is this correct? Also, noted the hw heater's drain plug appears to have been permanently installed; with teflon tape and now rust. In my former rv van, it's hw tank was drained by removal of this plug. Wondering, before I do something wrong, if this plug should be removed and serviced. At the same time, does or does not the petcock drain I refer to drain both units; ie, the fresh and hw heater????

Then, what are the 2 towards the back adjacent to the drainage pipes serving?

Thanks