If you pay someone to fix your vehicles, buy a new one.
If you are a DIY type of person and have time and place to work on a vehicle, buy used.
A DP will have engine in the rear and generator near the front. It will likely have air suspension and brakes, both of which are superior. It will carry more stuff including heavier materials used in finishing the house due to a stronger chassis. The towing capacity will likely be much bigger.
Buying new, you get the current electronics and state of the art engines.
Furniture, appliances, electronics, tires, flooring, and cabinets can all be replaced.
Engines, transmissions, slide-outs, floor plans, and length are pretty much static.
You will need to make the decision for your family's needs and wants.