Possibly because you just purchased the coach the previous owner could have left some "crud" in the tank my suggestion would be to pour a bag of ice cubes in the tank thru the toilet make sure you have 1-2 gallons of water in the tank then take it for a drive for 1 hour then dump your tank and then flush it out (thru the sani-flush connection) also check your gray water tank and put some chemical in the traps and fill that tank to 1/2 before your little road trip. On my previous coach an Adventurer I installed one of those cyclone vent caps and it worked well.
Hope this helps
Barry and Glennice, Kit and Kaboodle (Lhaso Apso's)
2008 Winnebago Tour WD, GMC Terrain toad