I have an '06 3500RL currently set up on my driveway and it's probably the same as slides go (I see no reason why some model year slideouts would be any wider than any other model). And with both slides out at the living room (on driver's side and on passenger side) the measured distance is 14' 7" (I would use 15'). The dining room slide goes out just about as far but there is no opposing slide on the other side. Finally, the bedroom slide does not go out as far as any of the other slides so I didn't bother to measure how much. I used a tape measure in the dark and with 55 degree weather, brrrr. I was only wearing a T-shirt and short pants.... I probably should have gotten a sweater, brrrr.
So with slides in, the width would be 8' (see model year brochures) and when extended the width should be less than 15 feet.
You don't mention what year you are asking about so send Steve Reigel a message, he had a 3400RL but I don't know what year. I am pretty sure the 3500RL has the same retracted and extended widths and that only the interior floor plan differs. I also don't expect the year to matter so I don't know how exact you want to be.