Sounds like you have a lot of fun ahead of you! I bought a 4-Wheel shell and did the whole inside myself. You don't necessarily need to use the same equipment the 4-wheel Camper uses unless there is size and fit issues. If you go to their website they have posted manuals for their various accessories like stove, etc. I would recommend you do a little Google search in your area of RV Supplies & Parts Stores (stores that 'specialize' in RV parts) or Marine supply stores to get these kind of small specialized parts likes stoves, refrigators, pumps, etc. I didn't put in gray water tank so I cannot advise you. Didn't want the weight. I have a hose spicket that goes out the side wall behind sink and empties into a small bucket. One idea that I did have though was to use a large diameter PVC pipe and put it along the outside bottom wall with a valve at the end. Good Luck!!
Thats a great Idea Yukon thanks , may put a ball valve on there and get one of those collapsible buckets that fold flat ! ,D