Are you sure your vehicle will really tow 5500 pounds? 1/2 ton pickups, which claim to be able to tow 7200 pounds in reality generally should be kept below 6000 pounds. So if the 5500 pounds is a dealer claim, there is a good chance it is inflated. From what I know of the Astro, I would not be surprised if it only can tow 4000 pounds, or even less, safely.
This is bad news from a travel trailer point of view. As is mentioned above, a pop-up may be your only choice.
As far as price, pop-ups are probably cheaper than travel trailers. In travel trailers, to get your price, you will probably need to buy used, or perhaps a new one which is 'last' year's model. For all practical purposes, you can treat a one year old model as a used trailer when bargaining for the price.