This is a list of things I highly recommend. I paid for each item, and I don’t get any freebies, so if it is on this list, it means I really like it and I use it.
Drying Mat – This fits right over the glass stove cover in my Alto, protecting the glass from being cracked by something heavy being dropped on it. Just trying to prevent accidents here, people! 
And it is a pretty good drying mat, solo or with the small collapsible drying rack I have.
Elfa Drawer System (1743 only). Someone in the Altoiste FB group figured out that turning this drawer system 90 degrees means it fits perfectly into the closet above the fridge. It’s brilliant, and by far the best solution I’ve tried in that space. Be sure to get the seven-runner size, that make the most of the space. And the top shelf option is useful for store awkwardly sized objects in the curved space at the top of the closet. (The link shows the system components that worked for me, and you can mix or match drawer sizes; I have two deep drawers and 3 shallow drawers.)
Fan (12V): The Alto has the built-in Fantastic Fan but sometimes, when it’s really hot and humid and you’re using battery power, a tiny 12V fan is the answer. I’ve used it a lot this summer, both when I am sitting around and when I’m trying to get to sleep on muggy nights. (I don’t use the adhesive mounts because I move it to wherever I am, inside or out).
Luci Lights are solar-powered lights that really do light up the night (or the inside of your trailer). You can get small or big clear Luci lights or splurge and get the colored Luci light that can cycle through the colors or let you pick the color that matches your current mood. The image above shows all the colors you can choose to display in the colored light. So fun!
The only ding on them is you can’t charge them via USB, which would be nice (I have other solar lights that do work this way for times when the sun isn’t cooperating). But put them out on a sunny morning, and they will be fully charged by afternoon and that charge will last 3-4 evenings.
MacBook 12V charger: For those times when you don’t have shore power, this little plug does the trick. Make sure you buy the right power level, my MBP uses the 85W version.
Rumpl Fleece Puffy Blanket is the warmest thing on earth outside of a sleeping bag. It has ripstop nylon on the outside, and cozy warm moleskin fleece on the inside. And it keeps me warm when the trailer gets down to 55F and the outside temps are in 20-30F range. I’ve had several iterations of bedding and this is the one that made the cut.
RV Holding Tank Treatment: This stuff is great. It’s non-toxic, organic, and it works. It is a powder, so no worry about liquid spilling. It does the job. I won’t use anything else in my black tank.
Shoe Organizer: The mesh in this one means it can accommodate wide running shoes or strappy sandals without bulging out the top. Cut it down to the size that fits your needs; I have mine cut down to 2 rows so it fits under the shower curtain holder in my 1743.
Table Drawer: This shallow table fits under the standard Alto table (one side or the other) and it’s so handy to have a real drawer for pens and notes and crap that otherwise gets left on the table. The only modification you’ll have to make is to purchase shorter screws from your local hardware store; the screws that come with the drawer will pop through the table top if you use them 🙁
Teapot: I love this collapsible little thing, because I don’t drink a lot of hot beverages, but when I want one, I can make it easily without lugging a big teakettle around all the time. It folds down to about 1.5 inches high and the handle unscrews to fold down along the base. Cool device, literally! Only the bottom gets hot because the rest is silicone!
Toilet Roll Holder: Stays in place with suction and it is way easier to reach than the default holder provided with the Alto. ‘Nuff said.
Towel Rack: This nice-looking bar has amazing suction and I put it on the aluminum wall in my bathroom (it would not hold on the fiberboard (white) wall at all). It’s really nice to have an actual towel rack instead of hooks. The two suction points can be adjusted to any length of the bar, so it’s very flexible as to placement. And it comes in 18-inch or 24-inch lengths, so choose whatever size works for you.
Travel Map (US States): This fun sticker map fits neatly onto the inside of the Alto door and it’s fun to add states as I camp in them. What’s not to like?!
Trimetric Battery Monitor (TM-2030-RV): Having data that shows how much battery I have left, as well as how many amps I’m using, and the charging rate has completely changed my understanding of my power usage and storage. I had friends install mine (I avoid as much electronic work as I can), and it took them under an hour to do.
Vacuum Cleaner (updated Nov 2017): This little thing works really well in an RV. It runs off the 12v battery, has useful attachments, and is the only thing that reaches into the window sills to get the leaves that get stuck in there.
(Note: I am a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com. I get a small fee if you click through to Amazon items and purchase them, so thank you if you do that!)