Plenty of places to store a pack n play. Setting it up is probably only on top of one of the beds.
Never heard of wifi in any US RVs. Remember that in either case it runs off the cell network, so it may or may not work when your phone doesn't.
It depends on the type of RV you get. The ones without slides would probably be too narrow to put up a pack n play. Without slides, you have a fairly narrow passage from the front to the rear of the RV. If you get something with slides, it becomes very expansive once the slides are deployed and you should have enough space to set up a pack n play.
If you get a class C RV, you can sleep in the bed over the cab and probably set something up on the rear bed - but I'm not sure setting up a pack n play on top of a mattress is a good idea.
I do not remember if there is an option for a wifi hotspot from any of the RV rental companies we rented from in the USA. Never needed it as I can just tether from my cell phone.