Thanks everyone, this is more helpful than you know! We're looking at the G15TB in this line up:
http://www.forestriverinc.com/rvs/travel-trailers/rockwood-geo-pro
This 15'10" trailer has an unloaded vehicle weight (UVW) or dry weight of 2483 lbs and a cargo carrying capacity (CCC) of 1,383 lbs. By adding those 2 numbers, your get the gross weight or gross vehicle weight rating (GVWR) that you'd be towing of 3,866 lbs. The tongue weight would be a minimum of 10% of the GVWR or 387 lbs (10% x 3,866).
or something in the larger lineup from these guys with the bunk beds and such, the 16BHS to be exact:
http://www.forestriverinc.com/rvs/travel-trailers/wolf-pup?RegionSelected=1
This 21.5 foot trailer specs are 3,097 lbs UVW plus 780 lbs CCC for a GVWR of 3,877 lbs and a tongue weight of 388 lbs.
We were hoping to pull them with a Subaru Ascent or a Diesel Jaguar F Pace, though the Jag seems a little far fetched.
First, you don't need a diesel to comfortably tow either of those trailers. But, towing with a diesel is fine if that's your choice. Second, fuel consumption will greatly increase when towing any travel trailer. The Subaru has maximum towing rate of 5,000 lbs. As Gary RV_Wizard has noted you should not tow over 80% to 90% of a tow vehicle (TV) max tow capacity. 80% of 5,000 lbs is 4,000 lbs. The diesel Diesel Jaguar F Pace has a bigger max tow capacity. So, you meet the 80% to 90% criteria with either vehicle.
This is important. The dry weight (UVW) does not include the weight of the propane tanks, any water in the drinking and waste tanks, or additional add-on options. A major difference between the 2 trailers is the max cargo carrying capacity (CCC). The Wolf Pup only allows for a maximum cargo of 780 lbs. The Rockwood Geo Pro has a bigger cargo capacity of 1,383 lbs. Loaded with all your camping gear, clothes, water in the tanks, propane tanks, tools, hoses, waste hose, camping gear, and other stuff, it is highly likely that you'll have a cargo of over 1,000 lbs. Thus, I think the shorter Geo Pro would be the best option for your family of those tow trailers.
Finally, because both the Subaru and Jaguar are unibody construction, I'd advise against using a weight distribution hitch. Please feel free to ask further questions or ask about other new or used travel trailers.