Hey everyone,
Thank you very much for your answers. I've taken them into consideration and this is what I did with the available budget :
Power :
You were right about the weight of a 12V battery, going towards 5.5V and usb devices was the best mobile solution.
I used two 5.5V USB LPE6 chargers, Two 15600 MAH ANKER batteries (pretty big and way cheaper that the 20000MAH), a small 6000MAH battery and an ANKER 21W solar panel.
This solution worked pretty well but I needed a backup plan in case I couldn't charge enough with the sun.
So I also used a more modular solution: a buck/boost converter. Those devices can change the Voltage, buck lowers it and boost....boost it. It allowed me to make modular setups and use common batteries ( in this case I used "D" size of alkaline,) or any other kind I could find on the way and lower the voltage to 5.5V.
I was also hoping to use this device to power my 5D directly from the 5.5V battery (to avoid the energy loss of charging the LPE6) and bought a DR-E6 DC coupler and all the AC powering kit. This didn't work because my buck/boost converter wasn't powerful enough.
In order to make it modular I needed to be able to change the outputs of the buck/boost converter (USB A/B , male/female etc...) So I took a set of connector plug.
Backups :
I needed a safe and light way to make my backups. So I decided to use my new cellphone (a Huawei Honor 8) as interface between my hard drives. It's using USB C so I took two USB3.0 FEMALE TO C MALE and a 124 GB micro SD card for the phone.
How it went :
I spent 9 days shooting up there, trying to use the solar panel and 20 alkaline D batteries as backup.
The two 15000MAH power bank lasted 4-5 charges on the LP-E6, I tried to use solar the most and managed to get 10000MAH on good sunny days.
I had to use the buck/boost as well and charged the power banks or the LPE6 with 8 "D" batteries. They discharged pretty fast but I wasn't needing them too much. Also, 20 D batteries weight about 4.3 kilos (8.6 pounds) it's a lot to carry.
I had shared feelings about the LPE6 USB charger, they worked well in open air but heated a lot. I couldn't use them to charge in my bag during the day because of this.
The backups went fine but the copy was slow. I should have bought a faster Micro SD card, this one was slower that the USB 3 norm.
Also physical hard drives tend to draw a lot of power from the phone, the 2000MAH battery from the phone was holding enough time to copy 70/80 Gb... SSD's would have been better or maybe a system to power the HDD from an external source.
Hope this was useful or interesting ! If you have any question about how it went let me know and thank you again for the suggestions, it helped me a lot.
-Pierre