Let's see if we can help.
Where's the fan located?
Is the fan exhausting air out or drawing air into the RV?
Does your toilet hold water above the blade/ball?
Has the repair guy removed the toilet and inspected the seals?
Do you leave the dump valve open all the time?
A couple of generalizations.
If you have an exhaust fan on and flush the toilet, it will draw the odor from the holding tank into the RV. (I bet you already know that though.)
It sounds to me like either the toilet flange seal is leaking, the valve seal is leaking or the toilet shell seal is leaking. It could be a small crack in a line or the tank that seeps a little overnight and causes the smell in the morning. This would explain why you can't pinpoint what's causing it.