does Drain order matter?

the plumber came yesterday to look at our basement bathroom remodel. Currently, the shower and lav join together and then into the 4 in drain.They are vented on an AAV on the lav. The toilet then joins the 4 in pipe immediatley downstream of the wye for the shower and lav. No more vents.

We are wanting to move the shower to the other side of the toilet. The plumber said he could tie into the drain downstream of the toilet for the shower. He suggested either a new AAV on the lav, or tieing a vent from the lav into a vent stack that is about 10 feet away. But again, only the one vent.

It seems to me that I have read on this site that this kind of set up will cause the toilet to suck the shower trap dry. Am I understanding this correctly? If yes, whats the solution? I cant see a way to tie the shower in upstream of the toilet. A vent on the shower too?

Normally, each trap is vented. The vents can be tied together so you only have one exiting the roof, though if done high enough. An AAV is not the preferred way to vent things, but can be made to work in certain situations. One of the pros may have some better ideas, but it would be helpful if you could provide a drawing or picture of the arrangement, changes you wish to make. They would also probably need to know the sizes of the existing piping. My unprofessional opinion.

thanks jim
i can get pics tonight. But for now, its all 2 inch pipes.

The toilet must be at least 3"...

drain

Jad, it appears your internal date on the computer is a few months behind. The only way to connect the shower after the toilet is if it has its own vent to equalize air pressures.

So here is what the plumber proposed. His company has done work for us before that we are quite happy with, but this particular guy didnt seem to sure how to tackle this job. He is also not who would do it.
So if the vent was on the shower, can the lav be wet vented, or do we need vents on either side?

Help, please! I need to know if I need to find another plumber, or if this is gonna be ok!