Knowing what I now know from building my first trailer ?
1.) The first change that I would make would be to get 4 smaller batteries @ 100 amp hours each and figure out how to install them in the center of the trailer over the axel and set up the batteries for 24 volts. The reason for this is simple math, because you can have 2 times as many solar panels connected to your single charge controller. 12 volt batteries = max 1250 watts. 24 volt batteries = max 2500 watts. 48 volts = max 5,000 watts. Be sure to add some battery venting. Now while many may say this is an insane amount of solar panels, let me tell you that my larger system also charges and operates everything in the almost shade, cloudy overcast or even during the rain. I notice the panels coming to life at 7 am in the morning until late in the day. The panels weight is only 40 lbs each. I don't need to climb onto to anything to tilt my panels, maybe just change the direction the trailer is facing. One of my goals was to have a small efficient 100% solar operated a/c system. This is not possible in the RV world as of today. I have noticed 30 volt house panels are many times cheaper then 18 volt RV panels. The components are higher quality, safer to operate, more dependable & if I decided to leave the road then they could be installed / function on a house.
2.) The second change would be to have the trailer designed to be better insulated starting with my ideas from the factory floor. No screws on the exterior siding as the VHB 3M tape a $400 option also stops - reduces the heat from transferring through the trailer ribs. A good amount of heat is transferred from the ribs in the side walls and the roof. I would then install a second layer of insulation inside of the the trailer a layer within a layer. This option would maybe add an extra $500 to my design.
3.) Better insulation, bigger solar & batteries would allow me to install a 9,000 Btu mini split inverter system with R410A freon. The advantage here is that it does both heating and cooling. These are rated at SEER 24 - 26 vs. the standard ac rated at SEER 10, compare that 10 MPG to 26 MPG and you get the picture. These new mini split systems operate from 10 deg F up to 115 deg F. The normal RV boondocker - RV is limited pretty much to maybe 40 deg to 80 deg unless on generator power or plugged in at an RV park. Besides who wants to get out at 40 deg or below & crank up their generator , fill up gas or suffer inside with 100 deg? Many RVers say that america shrinks 80-90% during the winter time, this would help solve this problem hopefully.
4.) I would not get drop down axels which lower the trailer for the rear ramp door 4" - 6". I am currently researching torsion axels at 22 deg negative. One thing critical is that the trailer must be towed close to 100% level. I would also get 2 x 5,000 axels as simply they do not cost much more but add extra safety to the trailer and towing. This would be great when going off road like BLM lands. Just like solar one could never have too much suspension in my opinion.
5.) Currently item 5 escapes me, but I'm thinking to expand the electric system, I will come back latter and add my thoughts into this post.