-Trailer and equipment maintenance maintenance maintenance x100 or your own laziness will bite you in the ass.
-Check suspension hardware, lug nuts and tire pressure periodically.
-Store the 7 way plug face down to hopefully deter moisture from corroding the terminsals.
-Inspect and clean 7 way j-box internals peridocally. Clean and install diaelectrc grease.
-Volt gauge on the trailer battery is nice.
-I installed a manual circuit breaker between the battery and inverter on my camper to lessen potential for a slow drain.
-Carry atleast one spare hub and brake assembly along with lug nuts.
-Provision to carry more than one spare tire. Having one spare is good but if you run over some gnary shit or if one blow out causes another, you're fooked with just one spare.
-Have dedicated "trailer/camper" tool bag / kit.
-Take the time to initially wire the trailer right and/or make wiring repairs correcrly. Don't hack it, let the wiring rot, or your own laziness will bite you in the ass.
-No sharp-pointy corners-edges under side of trailer. Your head will find them when you're under the trailer in the dark. Even more fun when it's cold.
-Inconspicous locaions where you can grind paint off/repaint easily for welding ground points.
-8 beefy tie down points / tabs per vehicle & locate tabs so they don't cause hooks to bind on the ratchet strap webbing when tight.
-Solid tounge jack. You're going to rely on it often so don't chince out. Carry wheel chocks too.
-Dedicated floor jack lift points
-Provision for vice
Check out my recent trailer rebuild thread. You might get some ideas from it or find some things that do and don't work for you.
https://irate4x4.com/tow-rigs-and-trailers/132770-death-metal-trailer-beater-18-trailer-refresh
-Andy