It's all in the head . . .
In the "newer" models, the main difference in the 30 vs the 31 is the configuration of the head (aka the bathroom) layout. Don't remember exactly, but think that the 31 has the split bath layout with the shower on the curbside, the spiffy & sink on the streetside. The other (30) has a single room that contains the shower, sink, & spiffy.
We prefer the split bath as it seems to have more room for dressing & stepping out of the shower, better access to the spiffy if another is showering.