Your treehouse sounds really cool.
If you want to give a dog a home, but can only do so temporarily, contact your local humane society or animal rescue and ask about being a foster parent for the summer. Shelters are stressful for dogs, and having a temporary home while looking for permanent adoption can really help their health. Plus you get a friend for the summer, and your local shelter isn’t stretched so thin.
If you can build a fence around your treehouse, that would give your foster dog a protected area to run off-leash and would also give you one more barrier (or a warning sound) before visitors and strangers got to your doorstep. Fences can be very inexpensive and simple, or complex and expensive, depending on what you want.
I agree with what other folks said about floodlights. One on each exterior corner of your treehouse will eliminate the possibility of anyone sneaking up undetected.
Hunters will be warned away by the color orange, in most places. They wear orange vests so other hunters don’t mistake them for animals. An orange flag or exterior paint might be worth considering.
For window shades, you can order custom sizes of roller shades from hardware stores like Home Depot or Lowe’s. There are also paper blinds that you can cut to size with scissors, and stick to the window frame. Like this.
Do the windows open to offer a secondary means of escape?
Really cool layout.
I definitely suggest learning the shortcut keys for SketchUp. There’s a HUGE difference in flow when you don’t have to keep going back to a toolbar.
Don't bother with a solar bag, they're useless. They're super heavy, you have to hang them really high or sit down to wash your hair. I don't like them at all. You might try a rechargeable camp shower. This is the one I've got. https://smile.amazon.com/dp/B0839FY2RD/ref=cm_sw_r_cp_apa_glt_fabc_T4MSZBXY490HF5ZBV5QC?_encoding=UTF8&psc=1
Looks like a well thought out design. +1 for using Epever charge controller, they've been pretty good quality without breaking the bank and you can use the RS485 connection on them to get information from them.
Those MC4 fuse holders are crazy expensive for a barrel style fuse. I'd just use a fuse bank inside the van for 1/10th the price or less that uses modern blade fuses that the vehicle uses.