Thanks everyone for the answers. I guess I did not word my question correctly.
We were on a shake down cruise and at a campground at Lake Powell. Before heading to Las Vegas for a night and then home, I drained the tanks, black tank first, very strong odor both inside and outside the RV, entry door open, windows open. With a slight breeze blowing and several windows open and the entry door opened and screen door shut, odor cleared in a few minutes. No odor from the grey water tank when drained.
Later, at home, I ran Fantastic Vent fan with all windows and the entry door closed. When I returned after lunch there was a very strong blank tank odor inside only. Fan could have caused a vacuum, but where could the odor enter the RV?
Once again at home I had the fans on and had to go inside the house, with 2 windows open there was once again a strong order inside RV, no odor outside RV.
It is my understanding that the vent system is a closed system inside the RV with the actual vent termination being outside through the roof.
So how would the very strong odor enter the RV? I checked the vent checks and they look new without any indication or backflow from tank, I assumed last night they were working.
I understand that if the tank is not clean and there is a pyramid of poop on the bottom, excessive paper hanging on sensors or any other problem caused by lack of good maintenance of the interior of tank, that there will be an odor from the vents. The vents terminate outside the RV not inside the RV.
In our previous RV, purchased in 1998, and driven to every continental state in the USA and all but three Canadian provinces without this problem ever occurring inside, that there are two problems, poor cleanness and an opening in the vent system someplace inside the RV. Lack of cleanness alone should not allow the system to vent inside the RV, unless there is a failure or damage to the vent system.
If these are not correct assumptions, please let me know what else could cause the problem, by the way, there was a water seal in the toilet and as far as I know, in the shower and the two sinks as they had been used lately.