We traveled in a 19' class B for many years. It was great for weekend getaways, and pretty nice for week long trips. It was ok for two week excursions, but became barely tolerable for three weeks.
After I retired, and started going on six or seven week trips, we just had to get something larger. Try to imagine being cooped up in one for two days of heavy rain. It becomes claustrophobic.
The one major problem with them is a serious lack of storage space. We usually ended up with loads of clothing and other stuff stashed on the bed. Of course, when we went to bed, it would all get piled onto chairs, and then back onto the bed in the morning. That twice daily shuttle would become a real pita by about the 20th day,
We ended up stashing a lot of stuff under the bed. That was fine, if it didn't gre too cold at night. It seems that the stuff under the bed blocked much of the air flow from the furnace vent under there. Of course, the reduced air flow wasn't enough to activate the sail switch, so the furnace wouldn't operate.
There may not be a separate bathroom sink, and if there is a shower, it will probably be a wet bath. In some class Bs, larger people may have trouble sitting on the commode with any degree of comfort. Try it out. In some units, the commode is in a clothes closet. You'll need to shove the clothes out of the way, unless, of course, the closet is jammed full. In that case, you'll end up moving some of the clothing to the bed. See above,
In many class Bs, the bed folds up into bench seats. These may not be terribly comfortable, and may not be long enough for some people. Make sure to try out the bed before you consider buying one.
Joel