Take a look at small bed-slide trailers like the 21-ft. Rockwood Mini Lite Roo 21RS. The hard-side bed (not tent bed like a pop-up) slides out when you set up camp and slides back in for travel. Another similar trailer is the Jayco Jay Feather X213. They both have the slide-out queen bed at the rear and a set of bunks at the front.
The Rockwood model has apparently been discontinued, so would only be available used, but the Jayco is still being built. The Rockwood is 21'9" long and has a GVWR of 5450 pounds. The GVWR for the Jayco is 5500 pounds, but is 24' long (counting the A-frame tongue). So, if a few feet of length is critical, you might want the Rockwood.
And manufacturers have differing numbering schemes. The Rockwood 2109S is 21'9" long from the hitch ball to the back of the rear bumper, ie., total length. The Jayco X213 is 21 feet from the front wall of the trailer to the back wall of the trailer, but you need to add 3 feet of A-frame tongue to that to get the total length.
We just sold our 21'9" Rockwood Mini Lite travel trailer (not a Roo, but that is the Roo's model name) yesterday. It is not the best-quality "superlite" trailer on the market, but it has all the features we needed including a full "dry" bath and a sofa slide.
But it was perfect for a retired couple to take to Alaska for 3 months during the summer of 2013 using a mid-size Nissan Frontier 4.0L V-6 as the tow vehicle. We added a second 12v deep-cycle battery and LED light bulbs for boondocking, but had no generator. With the batteries and the 3 30-gallon tanks (fresh water, GW, BW), but no hookups, we could stay 3 days at places like Riley Creek Campground in Denali NP, washing dishes and taking "Navy" showers every day.
Most RV manufacturers have very similar small trailers--Northwood and Lance being two that are very well-built, but heavier and more expensive, than the Rockwood. Try the craigslist RV ads in the nearest big cities--Dallas, Houston, Albuquerque, wherever--for used trailers from some of these manufacturers to get an idea of prices and features. Also check out the on-line NADA price guide for the models you are most interested in to get average retail prices.
Good luck in your search and don't buy too little trailer to provide whatever level of comfort your boys might enjoy.
"Retirement is the best job I ever had!"
2015 RAM 2500 4x4 crewcab 6.7L CTD; 2016 Rockwood Signature UltraLite 5th wheel