G'day Andrew,

That depends on personal comfort and the season that you're camping in.
Alot of people are happy to just sleep on the stretcher, but if you do alot of winter camping this can be quite cold.  
Adding an extra mattress not only provides extra comfort, but also insulation which will keep you cozy.
We'd generally recommend at least having a mattress in your camp kit so you can use that as an option if & when needed.

G'day Dave,

You'll loose about 45cm of floor space from the back wall with a stretcher/mat combo in the RV tents.  Given the tent is 260cm deep & the stretchers you're looking at are 190cm, you'll have a little bit of room to play with in that sense.  Just bare in mind width wise, you'll only have about 10cm or so between them so there will be little to no available floor space for storage.
Hope that helps give you an idea of how it would work.

G'day Jacob,

Fully pegged out and tensioned correctly, you will be able to fit them front to back in an RV4, but definitely not an RV3.