It is about box size. Look for twelve foot or longer box. Box length is often in the model number.
You almost always get a king and a queen (RV sizes, not actual household sizes) with a 14 foot box, and it can also be two kings.
A twelve foot box can just fit two kings, but more often it is a king and a queen. A lightweight 12-foot might be two queens or a king and a double.
A ten foot box can handle a king and a double, lightweights more often will have a queen and a double or full.
Get down to an 8 foot box, there is room full bed and a twin.
Issues are more than the space available to push in the beds. Larger beds are heavier, need heavier supports, and there is a matter of balance when the beds are occupied, whether there is enough leverage from axle to rear jacks to keep the weight on the rear bed from tipping the PUP or bending or burying the jacks.
Tom Test
Itasca Spirit 29B