I'm an only-parent with one teen, doing similar research. Here's what I've got so far, from a mortgage broker, a real estate agent, municipal planning rep, etc.
1. Like gardeningandgreen said, many lots in my last area came with stipulations -either the developer's or the (rural or municipal) government's. Build within a year, no RV parked on it until a large conventional house is in place, etc.
2. Sometimes the stipulations are kept quiet until you require them with an offer, so do require them.
3. Ensure the lot has RV access -not necessarily a paved driveway or anything, but an option for physically moving the RV from the last real road to its landing pad. Grading for this can cost.
4. Realtor there suggested $80-$100k (Cdn) for getting lines (sewer, etc) in. Even if you won't need them, the govt may require them.
5. Inside my last city's limits, no septic line was needed; outside septic was needed (with back up field, etc). Not sure if an RV (vs build) would require either. I know one person living in a nonconventional thing, who carries their family's waste to a public park washroom once a week.
6. If if a cold area, child protection may do a song and dance re: living in a colder area, even if you are smart enough to figure out how to do that in a way that kids are safe and well. You may need to consider unnecessary extras for "appearances."