Downsizing from a 30' TT , you are already used to having a nice amount of room, having full time sleeping, dinning and sitting space. Down sizing to a 24-26' MH might not give you all 3 without breaking things up/down.
Look for a cabover full queen 60x80 bed, they are common and will be fine for the 9yr old, plus have extra storage space.
Rear queen full time 60x80 or sometimes 60x75 a short queen bed and a comfortable dinette that you can sit in for hours if need be.
They are not very comfortable in the seatbelt driving mode where a growing 9 year old will be traveling. I would be looking for a sofa or recliner option with seatbelts along with a separate dinette. That might take to you in the 28 ft range.
E450 will give you more payload over a 350. Get a 450 or 4500 GM.
Depending on rear tongue weight of the boat and rear axle rating, the bikes might have to go inside the boat or on a front 2x2 hitch and bike rack.
I would RENT a class 'C' first, to get a real feel for the 'C' opposed to towing a TT.
When driving a MH, you tend to hear things rattle inside and it will be a different ride than a TV and TT. It's a truck not a smooth suv towing a TT.
You might also need to watch the sometimes low rear overhang of a class 'C' on driveway aprons. Watch your hitch height and setup. You might need airbags.
Inside, its going to take some time to foam-pad things so they are quiet when bounce down the road .
You need to drive one and live in it for a few nights. Then decide what the pros and cons are regarding floorplans that will be specific to your needs.
Its all about having a comfortable floorplan to fit 'you'. A few nights rental money might save you from making an expensive mistake. Just something to think about .
