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?