Hi Albatross, we bought our wee van (a 1993 eldis Tornado XL) with the intention of doing some wild camping. Its early days and we have only had a couple of nights away wild stile. We spent one night in glen coe and the next night at loch earn without any trouble.
from our first three days and two nights away I can tell you that the limiting factor is going to be water. We used the full 55L we took with us in that short time. The toilet would probably have lasted another couple of of days before needing emptied or maybe more with careful flushing.
you will be limited to the amount of water you can carry by the weight limit of your van and will have to take great care balancing it all out to keep a safe nose weight.
I think we can safely get buy on 25L a day/night if we are careful with our usage and I am planing on fitting a trigger shower head to help reduce waste when showering and we are going to carry most of the water in the car since irs a land rover with plenty of space.
power wise we took an extra battery with us which I didnt have time to hook up in parallel with the existing battery but it did power the tv with an inverter and a 12v hair dryer and straightners for the trip. I am intending on connecting it in parallel and adding a 100w solar panel which I recon will about keep up with our needs.
If you haven't seen them before theses sites are quite good for finding places to stay
Wildcamping.moonfruit.com
wild about Scotland