Just looked at your gallery over at stalking light WoW great stuff. I asked because something I hear from a lot of people out here on the road as an afterthought is "why did I put this huge inverter in, it sucks all my power". Microwave aside everything you listed actually steps down the power to 12 volt to be charged. Camera batteries between 7-8volts. Laptops range up to 19 volts.
If you are going to run a microwave or a huge blender go for it. Otherwise 300w should be plenty.
I run blue sea usb ports for charging things like phones, tablets etc. I do not have to use the inverter.
I use my inverter for sensitive equipment like camera batteries, laptop etc.
As far as length my inverter sits right next to my battery, about 15" of cable running to the battery. I also have a ground wire running to the chassis.
Congrats on the Eagle.