fresh water tank overflow syphoning tank dry

I find this thread interesting. Our fifth wheel also has a pressure fill and valves to determine city water/system use/winterize etc. Last year I started noticing that when I fill the tank to capacity it would drain WAY longer than it used to when tap shut off at house. I would get to campsite and notice my "idiot light" was showing 2/3 tank. Not trusting it I ignored it. Now I show up at a campsite and find I can add 1 or 2 6 gallon jerry cans to what I believed was a full tank. In addition our family following us on road trips notes that we lose water on left turns and not just a little...looked like smoke coming off the tires. The last time I topped off the tank I noted water coming out overflow but when I got under the rig there is actually two red "overflow" tubes with one coming out hard/fast and the other trickling out. I wonder why there is two tubes? Is one an overflow and the other the breather tube? I would really like to understand them and may have to pull the coroplast to see whats changed as it used to just drain off a gallon or two as the tank was full and somewhat pressurized.