Check your holding tanks

After owning 3 RVs this was a fist. Last trip ready for winter, had drained tanks at park rig was level came home parked on pad which is level. Always remove termination cap and open all 3 valves to air out. about a week I smelled sewer bad. Ok closed valves drove rig up on 2 2x8sfor better drainage. I put 5 gallons water in each tank then open 1 valve at a time They all over filled the bucket and lots of bad things also. My Keystone Avalanche is two years old and have traveled about6,000 miles never completely drained looks like. So you might want to check it out. I guess the tanks are high on the drain side
Thanks Keystone so now I guess its 2x8 time if I want it drained completely. Something you might check might just be a breading ground for who knows what?