Newly-filled Propane Tanks empty in four days-not used much

I am back, pretty sure I got my breaker problem resolved. Now I have a new one with my 2010 Cougar. I have two 7-gal propane tanks, one on each side of the front end of the rig. I use propane only for furnace and stove and seldom use either as I prefer elec/microwave/gadgets etc. I went to turn on the furnace during a cold spell we just had and it was empty. OK, got the two filled up and installed them. Forgot to turn on furnace at 65 degrees over Thanksgiving, so it did not use propane then. Since then I have kept it at 65 degrees and on but it doesn't come on because I have my space heaters going and they keep rig warm. So I use furnace only in the a.m. at 75 degrees, then down again to 65 and it doesn't come on. My point is that I have not used up any of the propane I bought just before Thanksgiving. The red marker shows them empty. We have used the soap method to check for leaks on/around the tanks. All looks good. Local dealer said even with bubbles those tanks would lose only a small amount if they were leaking. One person smelled propane near the tanks. Dealer also said someone probably stole the tanks by switching them with empties. I tried to turn on the stove burners and there is no propane in those lines. The stove has hardly been used. Two summers ago someone came into our RV park where I and many others live full time and stole the gas tanks out of three river boats during the night right under bedroom windows (it was windy/raining) so I guess someone could have switched. What I am going to do about this is fill the front/main propane tank again, install it, turn the secondary off completely so it won't transfer over, and see what happens. I did run the furnace yesterday a.m. for 15 min. That's it. Turned it on again this a.m. and the furnace started, air blew out, no heat came out, and furnace shut off. That is when I checked the tanks, both empty. My question, after all of that, is between the tanks and the furnace and stove, which are close to each other, could there be any kind of leak problems that would use 14.2 gal of propane up and I am still alive having been inside this rig most of the time? I can't smell anything due to sinus surgeries, so I have to depend on others. Thanks