My recommendation is that you continue to look at full offroad campers, even if you're not going to use the full capability. This means you get the build quality and resale opportunity of an offroad trailer, as well as retained value. The market for on-road campers seems much smaller.
That said, a Cub Spacematic/Supermatic might be worth looking at, and well within your budget though the teenager may wish to consider tent or swag, with the younger kids on camp bunks. I would have thought a Camprite would be over 75kg ball weight. From experience with a mate who has one they're heavier than they look.
How you pack and the length of the drawbar will have an impact on ball weight.