IMHO...
Negative miles are too few on either a gas or a diesel engine. In general I don't see unusually low mileage as a negative; there's basically nothing in an engine that wears out from non-use, just things that wear out from use (which high mileage has more of) or time (which makes mileage irrelevant). Some people do think it's odd to have to replace a ten-year-old widget on an engine with low mileage because they understand that widgets should last for more miles, but ten years is ten years regardless. In terms of high mileage, very few motorhomes given proper maintenance will ever wear out their drivetrains. The Ford V10 could reasonably be expected to go for maybe a quarter million miles without needing extensive work, and most diesels more. That's not to say there won't be some odds and ends that need replacement, but the basic engine designs are pretty robust. (Likewise, the transmissions used are generally pretty bulletproof if given appropriate care and maintenance and not outright abused.)
That many people in any RV will be more or less crowded. Whether that's a problem depends on the people involved. In many cases, you'll probably be spending a considerable amount of time out of the RV anyhow...maybe having some sleep in tents if the campground rules allow that, eating at picnic tables, sitting around under the awning (or out in the open), showering in the showerhouse, etc.
Do take into account that many campgrounds have limits on the number of people permitted per site, sometimes with exceptions for immediate family and sometimes not. Do also consider how much weight any RV you look at can carry, and how much fresh water and waste water, and work out how much that is per person. A class C with, say, 35 gallon water tanks is going to require a lot of conservation to go more than a single night with that many people before needing to dump and refill. Even a single night of dry camping would need careful usage. If you're camping with hookups, that's somewhat less of a concern.
It might be practical to tow a travel trailer, or maybe just a popup, behind a motorhome.
Renting an RV for a weekend or week would be money well spent, I think. It may well mean rethinking your dream a bit, but it's better to find that out after spending a thousand dollars than after spending thirty thousand dollars.