When I purchased my first RV, a Flagstaff 228 popup camper, the fine folks at Roberts Sales really helped educate me on how to choose the best floorplan for my needs. I have tried to capture a bit of what I learned below for the benefit of others. (RETROACTIVE EDIT: after owning our popup for about 5 years, I revisited this post and appreciated the information even more after enjoying over 100 nights of camping in our popup!)
Discharged water from your popup camper’s sink or shower should never be allowed to simply drain out the side and onto the ground. Food particles will attract animals and soap chemicals will damage the landscape. This is particularly important in desert environments where even clean water discharge can cause erosion.