Need Advice

Renting is an excellent way to see if your dream becomes a personal nightmare and what type of RV is best for you. Eight to ten people in one RV? Not me...ever. And you won’t be able to boondock more than one or two nights. Adult children should have their own RV or tents.

I think if money is already tight, buying an RV is the last thing I'd do. Even brand new ones break and cost a lot of money to operate. I'd use that $30,000 over the course of a few years and rent a house/cabin that can fit the whole family, and just drive there.

If you're dead set on RVing, I would definitely rent one first and do a trip for a week, even if it's just with your little kids. That will show you if this is something you're really going to be interested in enough to commit to it.

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.

8 to 10 is many. Have you tried to assemble all of you in one of the RVs you are looking at? How many of you will travel together in the RV? How many will have a seat belt available? Is that a legal requirement (it is where I live)? You will not only need to sit, sleep and eat in there, all your gear has to find a place as well.
Electricity, water and waste water capacities are limited unless you are hooked up externally or you can use campground facilities. Where and how are you planning to travel? If you want to boondock, you will soon run out of capacities.
I would not focus so much on Diesel or Gas at this point. For older RVs, potential issues are often not with the base vehicle (engine, axles, etc.). And if they are, they can usually be repaired without much difficulty (but may be costly). Older RVs can and frequently do have leaks that eventually cause rot. Frames can crack, slideouts can stop working, things like that. If an older RV is well taken care of and maintained, the risks are smaller.
What about a van to seat them all, a travel trailer, and maybe an extra tent for sleeping space?

x2 I just spent $3k on mine in the blink of my eye. rent an rv before you spend 30k on one. see if this is for you. or even cheaper rent a cabin in the state parks or by private owner.

If you don't have around $10,000 or more laying around to put an engine or transmission or both or whatever in it unexpectedly, don't get into rving.

It's also expensive to pay someone to repair most RV items. With RVs they say be handy or have a lot of money.