what you seem to have there is one step up from a "teardrop" trailer....small and low...both of those are advantages IMO if it fits your needs....that's the definition of a perfect trailer: the smallest/lightest one that fits your NEEDS....congratulations if you found yours
I agree with your 100W decision....panels cost so much per watt...and the panel will only be a part of your whole system (panel, controller, fuse panel, battery)....you've got the room on the roof so why not go with the bigger option...you'll never regret that but you might regret (and be back at the store) if you find that 50 is just not enough
the actual dimensions of a 100W panel might not be practical/desirable for your roof size/shape...they come in all different sizes however so you could keep looking or get an 80W if it turns out to be the best fit for your roof....maximize whatever will "work"
Panel prices vary WIDLY....IME they can run from as low as a dollar a watt (a steal you might run into especially seasonally) to as much as three dollars a watt.....so SHOP
Given that your trailer roof is low (read easily accessible) I would mount the panel on the roof "semi-permanently" ... there's lots of trees in the world and if you happen to park under one the panel will be useless...if your mounting system allows for easy removal (wingnuts) you'll be able to take it off easily....a plug connector near the panel and an extension cord will allow you to go place the panel not only in the sun but at a perfect angle to it (MAJOR improvement in performance compared to a panel on the flat even if you were parked in the sun) Given that your trailer is small you will probably have a single 12V
battery with X amount of storage (amp hrs) so daily performance should be important to you
You could spend a substantial amount of money on
solar equipment that will give you lots and lots of info (percentage of charge, current power use, current charge rate, etc etc), interconnects with your tow vehicles
electrical system and automatically powers the AC side of a trailer...at the other end of the scale is a panel, connected to a simple charge controller, connected to a battery (no 12V power from the tow vehicle)...if you choose this route the only information you will get from your system is when it's time to buy a new battery because you have ruined said battery (no longer holds a charge)
A rather inexpensive meter will supply all the information you actually NEED to maximize battery life. What I use is a permantly mounted 4 digit panel meter that can be checked at a glance any time you want (as opposed to a handheld meter that you have to get out and connect to your system now and then)
12.65+ volts is a fully charged battery
12.2V is 50% discharged and the level you should not let your system get below because you will be damaging your battery (shortening its life)
12.4V....if your system can stay at or above this value you will be maxmizing the rated cycle life of your battery (the rated amount of times it can be charged and discharged before the end of its "natural life")
(all these values are measured with no load on, batteries at rest for like 30 to 60 minutes)
great hobby this
solar stuff... keeping it simple is the hard part
You have a neat, rare trailer there....have fun, good luck, F
pic of a dumb charge controller, a slightly "less dumb" controller, a 4 digit panel meter and "a shady spot is a good thing to have when it's HOT"