I was running RTT on my XJ since our first trip to northern scandinavia back in 2000. I´ve had like 4 or 5 of them, bought them all used and sold every of them with some profit. The following text is written from a european point of view, I do NOT know the US market.
The first decision you have to make is if you want a RTT with a "hard case" or a foldable one which is more built like a classical tent.
The hard case modells are usually more expensive and you can mount them only over the whole length of the roof of the XJ. So you sleep entirely on your roof, they usually have an usable length of about 210cm (somewhere around 7´) and come in different widths between 120 and 170cm. There are manufacturers who make longer versions of their tents with a usable interior length of 230/240cm.
The foldable tents are only half as long and reach their full usable length when unfolded. That makes them quite more flexible in how to mount them. You can unfold them sideways - so you get a small usable room under the tent, beside your XJ. The ladder is used to support the free part of the tent. You can mount it so it unfolds backwards - but then it interferes with the hatch of the XJ. On other vehicles with one or two doors opening to the side this is a perfect place since you get a nice place to cook that is well protected from the elements. Or you can mount the tent to unfold to the front - then you have to have a front bumper like from ARB where you can step on and where the cut down ladder is put on - I´ve seen this solution only once on an XJ - The advantage is that you can relocate the vehicle (to put it on a flatter place for example) without having to close the tent.
The negative side of the foldable tent is that you have a cover which has to be stored somewhere overnight. This cover gets wet, dirty and full of dead insects (at least at german autobahn speeds

- and it´s a hazzle every morning to put it back the tent, especially when you travel alone and own a lifted vehicle. On the other hand - some morning exercise hasn´t hurt anybody... (even if I prefer my morning exercise in a more brunette or black haired female style, without any dead insects anywhere, thanks!)
Regarding the mounting options: Forget the OEM "roof rack thingy" on the XJ. It´s good for some lights, perhaps a shovel and an axe - not anything more. The only way to mount a RTT is using the rain gutters on an XJ.
There you can choose between the Thule stlye single cross bars (I think three are a good idea, they go for cheap sometimes on ebay) or a complete roof rack. Whereas the cross bars are less expensive and much lighter the roof rack offers more possibilities on mounting stuff like lights, vehicle plates, antennas, axe, shovel, fuel canister, camping table etc. But watch out - packing even more heavy stuff on the highest vehicle isn´t so smart in my book. I´d never put fuel or water up there (that belongs as low as possible in the vehicle) and I keep the stuff I put there to a minimum (some lights, camping table below the roof rack, license plate).
If looking for a RTT: Check if it´s possible to let your pillows, sleeping bag etc. in it while driving. If you have to put this stuff into your vehicle you aren´t getting the full benefit you can get from a RTT.
After my divorce I´m travlling alone again so I don´t my trailer anymore, that´s why I bought a RTT again. Since I work in the 4x4 industry now I got a brand new from Maggiolina for, well, not exactly cheap, but way cheaper than the MSRP. I have to say I haven´t seen a second one with such built quality! I was in the market for a "hard case" one and I´m more then happy with it. I arrive at the place where I want to stay, unlatch it, use the crank handle (under a minute) et voila - here is my 210x130cm bed with two pillows, a sleeping bag, integrated LED lights, storage nets, a flashlight etc. I think even a bottle of Jack could be stored there but I haven´t tried that yet.
