Like they do at the airports, you may have to weigh your family's luggage and restrict what they can bring! ;)
I just make sure they don't bring real heavy stuff, and that we wait until we get to our destination before we get heavy food products and drinks.
Another thing to remember is that anything you buy on vacation has to count as weight on the way home. So don't buy anything like furniture or yard decorations (i.e. gnomes). We make sure we expend as much of the food/drinks before we leave, to keep weight down. Any remaining water in the storage and hot water tank is drained as well.