windbrave,
Google "Coachman Freelander 21QB". You will find it has most of what you seek (dinette/couch combo) along with a massive outdoor storage compartment for your child's light weight bulky outdoor play toys. There are longer models that are still shorter than your max length. There are many U-Tube videos made by RV dealerships, showing off their used ones for sale.
The Freelander is an entry level motor home. You will find them to be more affordable. Just so you know, the actual length of a 21QB is almost 24 feet. Not 21 feet as implied.
