Howdy, esjay!
Casitas are something I know a little about, as my bride of 50 years and I have owned two of them over twelve years. Towed all over the Soutwest and parts of the East. We helped with the early Casita rallies, including the first large Bluebonnet rallies, where we had over 100 Casita "EGGS" to show up!
We live only live about 75 miles from the Casita factory and I have been there many, many, times.
My recommendation is to buy a half-ton four door V-8 pickup with a towing package. Take your pick. You will probably get about 13 real world MPG towing, plus or minus a MPG. And you will have power to spare, plenty of family room and a larger, safer, towing suspension and brakes.
If you pull with a mid-size or smaller truck or SUV with a V-6, you are still going to get about around 13 MPG or perhaps a just a tad better, towing! Do not let a salesperson tell you differently. And you will not have the safety of a larger towing suspension as with a full size truck or SUV.
A fully loaded, fluid wet, propane ready, Casita will weigh about 3500 lbs. Tongue weight will be about 475 lbs. You will need a sway bar, which the factory will tell you. Plus an electric jack is nice, if you are old like us! Our favorite model was the loaded 17'Spirit Deluxe.
We still have a lot of friends towing Casitas,(EGGs)camp with them, and they would tell you the same.
If you want to check out a really good Casita forum, go to the Casitaforum.com. and do some research. The forum owner and his wife are friends of ours. They own a tremendously modded Casita and knows probably more about Casitas than anyone I know.
Best of luck. Casitas are a wonderful, tough, and durable little trailers that have a very high resale value.
Mike