If you have children and plan to do any dry camping, you must consider the toilet capacity.  We do not want our children to use the NFS pit toilets, and the toilet capacity is only 3-51/2 gallons, depending on how you charge it.  TM tells me that it is not possible to add a holding tank, as there is no way to vent it.  There are ways to dump at your campsite into your own freestanding tank, but we have found it to be a lot of trouble.  We like our TM, but with 5 people in our unit this has been our major problem.