My personal favorite is layout #4 though I would probably be happy with any of them.
I like layout 4 because it has or can have the most usable storage space. There are two properties to storage space, presence and access. When you use the dinette as a full time bed access to the dinette storage space is difficult. We have put drawers in our driver's side dinette providing easy storage access. In addition we have room for rarely accessed items like our electric heater or crockpot.
Though the water tank is under the passenger's side dinette, we store a drill, and other tools along with a collapsible water jug, rarely used stuff but not difficult.
We do have a full width cabinet over the rear dinette. Yes you will hit your head sometime, but it's a rare event. We hang a couple of pieces of red yarn from it as a reminder. The overhead cabinet also provides a place for more lighting
and for us a place to mount a TV.
We also have little shelves over the side windows
between the rear overhead cabinet and kitchen cabinets. Also layout four has 5 over kitchen cabinets.
One of the most important things we did was to convert our rear dinette table from a table for four to a table for two. It can still fold out to sit four, a rare event. By having a table for two it makes seating at the dinette easier and more useful when you're not eating.
It's a great debate in trailers, permanent bed versus temporary bed. We've gone both ways and have decided the temporary is more effective, at least for us.
We found with the permanent bed making in the morning was a pain because it needed to look good for the day, making it at night it just needs to function. Also I can make up the bed in about 5 minutes and about the same time to take it down. Neither of us ever spend time in bed independently of the other, we are synced to the same sleep schedule. If one of us needs a daytime nap there's always the couch.
We love having the front couch. It's really a comfortable place to sit or lie down and read plus it makes it easy to have another couple in the trailer without crowding into the dinette.
Layout 4 provides a lot of counter space and makes it possible to have a lot of storage. We've added two drawers to the passenger's side of the sink side cabinets, shelve storage for pots and pans. On the oven side we have a pull out pantry and have added another storage volume under the fridge
We have added storage under the couch. In the center opening we have two pullout plastic storage containers. Behind them we have two small tool boxes. On the passenger's side of the couch we have added a large pull out drawer.
We do not use the couches bunks for sleeping. As a result we have unhinged the back and removed the back's plywood. This allows us to store additional mattress material for the bed while reducing weight
Since we don't use the bunk feature we have added a large cabinet that hangs off the bathroom's front wall. This deep, wide cabinet is shelved and holes a good share of our clothes.
The tall standing closet has been reconfigured with a two doors. The bottom of the closet contains our air conditioner. Above the AC is a hamper for dirty clothes it holds a weeks worth for two. Above that the closet is shelved for box storage, cereal, rice,.....
We also have a fair amount of outside storage, 5 under trailer storage boxes and 1 large rear bumper box plus 2 large 5" x 5" x 4' hollow containers.
As to the front sewer pipe, we like it. It is long enough to contain a sewer hose that covers about 80% of our dumping needs. We have a second container for a longer hose when we have to go a longer distance and the two hoses can be tied together.
If I were trying to minimize rivets I'd start with the curtain rivets. I think there are 24 of them. In our case since we have wood shelves above every window we can hang the curtain rods from the wood. However we bought the trailer used and the rivets were there.
Our trailer is now 23 years old and we have not had to replace an outside rivet yet. I can not say the same for the rivet caps.
Choosing a trailer has a lot to do with how you plan to use it. We travel a lot and need storage, particularly organized storage. We do not pile things in nor do we carry coolers or other loose items in our trailer. Rather we have added storage.
As to grand kids, we have traveled with ours. It's a rare happy event. Fortunately or maybe unfortunately they have lives and there is really only a small window where that's possible.
As I've written many times I find traveling with pets a burden for the owners and others. It's easier if you're an occasional camper but if you're like us and on the road for 200-300 days at a clip it's limiting, often unfair to the pet and neighbors. (I recognize that pets can be loved as much as humans and am not trying to fan any flames.)
We're typically on the road for 6 months followed by a 2 month trip, sometimes we're gone for 10 months. I just can't imagine what to do with a dog(s) in our case. My focus is Ginny and me her focus.
If you want to see our trailer take a look at Preparing a 1991 Scamp under Modifications.
The downside to layout 4 for many is bed width, 44". I believe 54" is available in the deluxe Scamps.
Choices, choices... it starts with figuring out what you're going to do with it...for us it's travel.