#1 - upgrading to 50 AMP service (100 amps in terms of real capacity)... I can't believe i made it 10 years in our prior trailer w/o doing this. However, with a wife and 6 kids, This was the first upgrade i did upon deliver of our current unit. On our current 3 week road trip, i warned the mrs to remember we were on a 30 amp site. Wasn't 10 minutes later she tripped the breaker at the pole (Vacuum/Microwave/AC). I added two dedicated 20 amp circuits at opposing ends of the kitchen to run the electric heaters from (or the capability to run multiple kitchen appliances at once)
*** this also makes it very easy to separate the loads you NEVER want to run on #2 below by putting them on one of the 120V legs and connecting the inverter through transfer switch only to the other.
#2 - Magnum 2000W inverter. See #1, only apply it to drycamping (be it state parks / walmarts, rest areas, etc)
2a - the remote control panel and BMK monitor to see how we're doing on power consumption/etc...
#3 - Upgrade to Trojan T105s for the extra amp hours...
#4 - 4AWG dedicated charge line and disconnect from the tow vehicle. see #1/2/3... Depending on how much we've discharged them, I can usually bring the batts to 100% SOC after a few hours on the road. I can run the fridge off 120 while driving, i can change a ton of devices (laptops/ios/nooks/etc) and run an extension cord to the suburban to run the playstation on the same long legs of our cross country trips. I also have an outlet in storage to run the Milwaukee 18V charger for the cordless drill and impact driver - all while driving.
#5 - Fantastic fans + endless breeze - keeps things bearable in walmart parking lots when we're making a jong jump from destination A to B.
#6 - Honda Generator. For when we draw too much power and have too short a trip to replenish from #4. Quite and reliable...
#7 - EMS-HW50C - lets me monitor shore line power consumption on both legs...
#8 - momentary push button in tow vehicle, lines, + a couple relays to flash the parking lights on the trailer to thank folks for headlight flashes when changing lanes... (flashes off or on depending on whether they're currently on / off). Honestly, I've slowed down with the new trailer and don't do very much passing anymore, but it comes in handy even when navigating in town/etc where interstate lanes shift/diverge...
9/10/11: Pending on current unit:
EZ-flex, underslung flip, Maxxis tires (how i hate Good Year Marathons - but i just bought another set of 4 as no one carries maxxis when you need em)!
Additional/Misc:
quickie flushes, 12 volt outlets by the wheels for portable compressor...
Bonus:
If i could finally completely sealed the underbelly and keep rats out of the darn thing, that might tie for #1. I've pulled resealed it twice and they're still getting in. Third time's a charm?