Do you have any storage solution suggestions for a very small trailer?

One option would be to rent a small storage space nearby. If allowed, they might also build a small storage shed on the property. It would depend on how much of a DIYer your son and his wife are, and how safe the area is.

Every available space needs utilized if you are like me. We put a bench in the hallway behind the front door for boots, shoes, gloves. It has a seat on it to sit down and remove winter items etc. On the wall behind the bench we put 5 or 6 hooks to hang coats, sweaters, school bags, back packs, etc. Above that we put binds on a shelf for gloves, hats, scarves etc for winter and summer both. The hallway is 38 inches wide and it takes up about 18 inches is all, so there is plenty of room (nearly 2 feet) to get by the coat area to the children's rooms and bathroom. Most adults do not go that way anyway. WE put hangers on every door for such things as the ironing board, iron, shoes, hand tools, crafts, spray paint, etc, anything we needed. One door even has a clothes hanger extension on it for all my husband's suit jackets. We also put short book shelves in the hallway for children's books and photos. Then under all the beds we have flat long storage bins on rollers. IN these we store the off season clothes and switch out with the closet and dressers when the season changes from spring/summer to fall/winter. Our futon in the front room has storage room in it. We bought it for that reason. It also works as an extra bed when needed for company. One of the larger bedrooms we put floor to ceiling book shelves (narrow 9 inch shelves so they do not take up much space) to store more books, tubs of holiday decor, and sewing notions etc A net was strung across one corner of a bedroom for stuffed animals and dolls, but balls and light weight other toys also ended up in it. It worked out fine. In our pantry we put a hanging shelf, in our cabinets we put hanging lid holders and for the cutting boards and cookie sheets. Also, in the kitchen sink cabinet we have a holder for detergent, gloves, etc. Here is one tutorial on the kitchen that may help you. I also built a small island for more drawer and storage space. https://www.hometalk.com/diy/organize/kitchen/get-organized-in-the-kitchen-32632436?r=2 Hope this gives you some ideas.

Is this a travel trailer or modular home? Sometimes used interchangeably but not always correctly.