Sounds like you may have a vent problem where it isn't venting enough. When you flush another toilet or use a shower, etc., the flowing water down the drain will suck nearby traps dry. (the toilet bowl is a built in trap). Contact the manufacturer and installer and a plumber and have one of them fix it. There should be a warranty. May be a plugged vent or it wasn't installed right or left out entirely(which wouldn't surprise me with a doublewide).