How about using ^that^ on the back of the popup, assuming you can add the receiver hitch to the popup in a way that is structurally safe?
I agree that using one of these right off the back of the rig would add more tongue weight than something further back toward the axle of the popup.
What tongue weight is typical for you now and how does the popup tow? If it tows great now then I would do what you can to keep the tongue weight as close to what it is today. Heck, you could do some tongue weight tests before you get too far down the road and see what is acceptable to you.
I know for me with the 80, the tongue weight I end up with when I have just a full propane tank on the front of the trailer is about perfect. A little less than perfect with a battery in the battery box. With a battery, and two full propane tanks up front like I had for the trip to Area BFE for CM10, the popup towed like poo. On the trip home with 1 empty and a 1/2 full propane tank and the batteries moved inside closer to being over the axle it towed great again.
I guess what I'm saying is that even though per Toyota you can have say a max of 500lbs of tongue weight doesn't mean you are going to be pleased with how the camper tows...find the sweet spot and try to stay as close to that as possible.
Here is a fun read (start at page 12) --> http://www.faa.gov/library/manuals/a...-h-8083-1a.pdf
If you could find some of the inputs needed then you could use the same formulas used to determine CG on aircraft for your popup