Most trips require less "stuff" than you might think. So the first thing would be to minimalize the stuff.
If you don't have upper cabinets on the ends, they can me added.
Another idea is to go to the junkyard and get a few of those cargo nets from cars or vans. With a couple of hooks, these can be strung between open spaces and "stuffed" with "stuff".
Get a flat garment bag and fill it with extra clothing like T-shirts and underwear, and lay it flat or hang it in the tow vehicle.
Also one thing which is often overlooked is the inside quarterpanel storage in most SUVs and minivans.
Kids are generally shorter than adults which means they could each have a container placed in the rear seat footwell even extending under the front seats of the TV without interfering with foot room.
Use a cloth bag for underwear and socks or extra sweatshirts. then cover it with a pillow case and use it in place of a throw pillow or a sleeping pillow.