I highly highly highly suggest looking for a seasonal campsite already setup with a camper. They are often plentiful and very good values. Most of the time you need to go to the actual campground to find them for sale. Sometimes they are listed on craigslist though. In my area you can find a nice clean 40’ park model about 10 years old with a deck, golf cart, landscaping, shed for $15-20k. This is way less than half the price it would cost to do it yourself. In addition, the down time it takes building a deck, shed, landscraping, etc. Your new trailer is going to be considered used after about 1 year anyway.
Seasonal sites are not a bad thing. Just MAKE SURE you try out the campground first. Rent a cabin or stay all day/into the night and drive home after midnight. Usually the largest campgrounds are best for season sites. They offer more to do and have more fellow kids for your kids to play with. And it’s more likely you will make adult friends too. I would also suggest a seasonal site near a lake or rec trail, because you could tow a boat, jet ski, atv, snowmobile, etc since the camper is already setup. This would keep the older kids interested during the high school years.