Lori, Mildew smell, not good! To check for plumbing leaks, disconnect from city water, turn on water pump, make sure all faucets, ice maker are off. the pump should not run if all fixtures are off. Might take some time (even overnight) for the pump to cycle if leak is small. One other thought-leaky toilet. If you have a Dometic 310 toilet, it may leak between the china bowl and plastic pedestal. You'll see some water on the floor when you move the trailer from campground to next campground. Not really a mildew smell, if you get my drift. Dometic had a run of 'leaky' toilets around that time (2010-2011). I called Dometic and they paid for a new toilet and admitted they had a batch of 'leakers'. You will need the serial # of your toilet to check.
Bill, Kathi and Zorro; '05 Beaver Patriot Thunder
2012 Sunnybrook Harmony 21FBS (SQEZINN)
2007 Jeep Grand Cherokee Overland