We owned a Jayco Greyhawk 31FS, sounds like the model you are talking about, for 8 years. We liked it because of the bunks for our kids, which were 8 and 6 when we bought the Motorhome in 2009. After a few years, both kids complained that the bunks were too narrow and that they'd hit the wall whenever they moved around. I believe they were 28" x 70". A normal twin bed is 38" x 74".
Eventually, I widened the bottom bunk so it was as wide as a normal twin bed. I couldn't really do anything with the top bunk. Our daughter moved to the cab over bed and our son used the newly expanded bottom bunk.
We no longer have the Motorhome as it was too small for us for long term camping. We camp seasonally now and we just didn't have the room. I work out of the camper, so I was always taking up the dinette with my laptop and other work stuff, so the only place for the wife and kids to "hang out" was the beds or the sofa.
I could not imagine living long term with as many people as you are talking about in any RV. Going camping for a weekend, sure, you can make due. Long term, I think it would be a hardship. We traded in our Motorhome for a 40' Destination Travel Trailer with a bunk house and 4 slides. The bunk house has 4 bunks and a trundle under each set of bunks and can sleep 6. However, the top bunks and the trundles are narrow. The bottom bunks are almost as wide as a twin bed and our kids are happy with them. They have had 3 of their cousins camp with us and all 5 where in the bunk house. It was a little cramped with their bags, but it worked. Again, I couldn't imagine that many adults even in our larger TT living long term. There is a half bath in the bunk house, but the hot water heater is only 10 gallons and after the 4 of us shower, there's pretty much no more hot water. I couldn't imagine if up to 8 adults needed to take a shower one after the other.
We estimated our Motorhome was about 240 sq ft, with both slides open and counting the cab over in square footage. We estimate our travel trailer is about 400 sq ft with all 4 slides open. Our garage at our house is 400 sq ft (20x20). Less than half the smallest single wide Mobile Home I've ever seen.
I would seriously suggest renting the Motorhome you are interested in buying first for a few weeks and live in it like you plan to do and see how it goes. When we first started camping longer term, we decided to start with a month and see if it would work out. It did, so then we tried 6 weeks before finally going for 4-5 months. After the first year of living in the Motorhome for 4.5 months we knew we needed something larger. Unfortunately, the TT that we bought last year was sold out at local dealers and it would be at least 8 weeks before Jayco could make another. So we waited until last year to buy.
Good luck.
-Michael