Well here's a few of my own thoughts and experiences since I'm looking to do the same thing - just because I have so much stuff I normally take anyway and a service bed would be PERFECT to store everything. Currently I'm considering a 4 wheel camper if I can convince them to do some minor changes.... The thing I would be most concerned about is most newer campers toward the tail gate are wider than 50" to accommodate things like a toilet. So getting one, or having a place custom build one, that is 50" the whole length (like the inside of most service beds) could be difficult.
I did find a thread on here somewhere where someone had a steel service bed and actually cut and re-welded the back 1/3rd of the bed to accommodate the camper. It was actually a super sweet set-up. But for me anyway, since I'm looking at aluminum, and new, that might not be a good idea since it could be a costly mistake to modify a new bed that I personally dont have proper equipment to weld (aluminum).
As JCatt said, the height is also an issue. It really depends on the vehicle. Most 'newer' trucks (like pictured) have pretty high bed sides, and on top of that most service beds for such trucks (that I've seen anyway) are even higher - like half way up the side windows. I'm sure you could find one to fit, or get a custom one built. For my particular truck (1997 OBS 350), most service beds are perfectly in line with the original height of the bed, so gap shouldn't be an issue.
I think what it comes down to is just making sure you get the specs of several campers, and several service beds, and see what would work best, and who might be willing to make some changes to accommodate (although most places that advertise "custom work" are anything but these days).
In the end though, I have found weight is a bigger issue. Depending on what your payload capacity is, and how much and what you plan on putting in the service bed - I have found that aluminum is pretty much a requirement, and with everything I have calculated, my truck will still be overweight. So that is something to seriously consider and research as well.
Another option I am considering that you might entertain is still a service bed, but ordering a custom fit canopy (not all that expensive, forget the company name), and then building your own quarters inside. If you take time, it could even be much nicer than most mass produced campers (like pictured). the problem here (for me anyway) is finding a way to cut the top out to make it a 'pop-up' (like pictured) so that overall height doesn't become an issue, especially when carrying 2 kayaks, and 5 or so antennas.
Hope this gives you some things to think about - I've been researching, thinking, making decisions and changing them again, for about 3 years on this topic alone.
