A little off topic of RV travel but pertinent to your family's travel plans. We have taken a family trip (us, our kids, our grandkids, and some in-laws) to Disneyworld ever 4 years since 2003 when our grandkids were very small.
Flying into Tampa is a lot less expensive than flying into Orlando. About $250 or so per person. But is about 1 hour longer drive, but easy drive. Tampa airport is a lot less crowded and hectic than Orlando.
Have stayed in Disney resort properties, off site hotels, and vacation rental houses. Disney properties are nice and convenient, but expensive. We have rented up to 7 bedroom, 5 and 6 bath houses for around $400-500/night thru VRBO for as many as 11 of us. Costs shared 3 to4 ways makes for a comparatively inexpensive stay. And off site means easier to eat at lower cost restaurants. Disney food is expensive also.
If they are just needing one room then a house would not be as attractive.
However they do it I know they will enjoy it. Especially if the kids are still young enough to thrilled by the "Disney magic".