Hi Nathane.
Just some thoughts... First re the propane tank, have you considered using a forklift gas cylinder... They have a fill point, vent point for self filling, a liquid outlet for LPG forklifts which if the cylinder is rotated 180 deg when on its side will provide vapour instead of liquid... (and if you find the right cylinder also a blanked outlet for vapour outlet). I know the laws are totally different here (and I'm not advocating undertaking anything dodgy or dangerous with cylinders), but I'm sure the logistics of portable forklift LPG cylinders are the same... If you are thinking of fitting a 'fixed' cylinder like under a car, I've been told some shippers won't ship with them. If you use a forklift cylinder, it can be removed, filled, swapped, left out if required etc etc. I have found 2 18kg aluminium cylinders here in Oz, and will be mounting them externally under the body. They can be removed, filled from a servo, filled at a fill station, and if you are in third world country's filled by hand using an array of adaptors....
Secondly... Re a detachable box, container twist locks and the actual container blocks are available to be purchased if required which gives you your secure legal base locks... Re lifting with a forklift, two pieces of 200 x 100 RHS on their side built into the box would work well and suit most forklifts..
http://www.conpargroup.co.uk/isocornercastings.html
Lastly re insurance, when we used to live in the UK, I ran all sorts of daft vehicles, including Land Rover 101's with 6 litre Nissan diesels shoe horned into them and also a few 'modified' tractors.. I always found the NFU Mutual to be very helpful. This was over 10 yrs ago tho and things may have changed, but I'm tipping your Unimog is on the system as lots of farmers use them for high speed road work.
Following your thread with interest as I'm in a similar planning phase.. Have truck, have a body, just need to make them look like they were always together! Re toilets, after a lot of research, and talking to several who have used them 'long term', it came down to the Natures Head or Air Head. I've settled on the Natures Head as it seems to win on feedback here. Good luck!