We upgraded about 6 of the 20 or lights in our Bambi. We just kept track of which ones we used the most and swapped them out for LED. This included the outside light (we like to leave it on in the evening), the lights over the table and the one over the kitchen counter. Thats it. All the rest of the many overhead lights, we never use (and they suck the battery dry).
We tore out the whole archiac 1980's TV, DVD, CD changer system which was a HUGE power hog in our 2005. An iPad with its little bitty battery is all that is needed. Still looking for a decent mount however to mount it on the wall by the bed.
Replace those bland white curtains with something colorful. Kate made ours herself.
Replace that noisy fridge fan with something quieter and more power efficient (still on my to-do list).
Buy a Link 10 monitor if you are a boon docker like us to monitor power usage and being efficient about generator usage. Our #1 upgrade!
Memory foam topper as per morgandc. Ok that was our #1 upgrade. Kate has ours arranged so she simply rolls it up during the day so the bed converts back to a couch (applies only to the David Winick version of the 19ft Bambi).
Inverter upgrade. Ok not so much an upgrade. But the Bambis don't come with an inverter. I have about 5 that are TOTALLY useless since they are not pure sinwave version. The crappy cheap ones (even the 400W units) can't even recharge an iPad unless your trailer batteries are at least 99% full. A good sinwave inverter will work down to 11V vs the cheapo modified sinwave.