I'm moving so my trailer is packed full right now with house stuff, but there's not much to see, just a big ole cargo trailer converted to a minimal cooking setup. For what I do prep area is more important than refer space or cooking equipment, and when you've got to move it's nice to be able to pack the trailer full of stuff and multi task.
Wood top utility table across front:
Table 
I added a commercial can opener to the table(~$60),
and another metal prep table down the solid side of the trailer(also from Sam's):
I wired the trailer with 3 interior outlets and 1 exterior outlet, all in waterproof conduit. Two 12' light strips provide light inside, and I may add outside flood lights next spring. I also added a 10K Btu LP wall heater (Northern Tool, on sale $89):
The only thing I plan to add for sure is an RV style exterior gas fitting so I can carry 2-100# LP tanks and connect them via quick connect to my fryer and pig cooker, and I plan to get a small gas convection oven.
I don't want or need (nor could I afford) a mobile kitchen, but this setup is easy to clean(car wash), will hold my pig cooker/fryer rig easily, and has enough room to hold a second pig cooker, plus my pop-up canopies, 4 8' serving tables, and a bunch of coolers for meat and stuff. Small cooking tools, mixing bowls, tea dispensers fit under tables in stackable cambros.
I just pull up to an event, roll off the cooker, and I've got a large prep space that's waterproof and out of the sun. I can offload cooker, coolers, and set up/tear down 2 canopies and 4 serving tables in less than 15 minutes, with little heavy lifting involved. Speed is nice after a long day, and limited lifting is becoming a necessity at my age. I don't have my 2 CS smokers in the trailer because I use them for smaller office parties and such. The trailer is used for church events or larger corporate cooks. I can do ~300# of butts or ~125 chicken halves at a time.
As it stands with one cooker, with some planning I can easily cook pork and either chicken or ribs for about 700 people at a time. The second cooker I plan to add will be used mostly for chicken, because when you need chicken, you need a lot of it at the same time.
I'm not sure what your interest in trailers is related to. I don't remember if you're looking to compete or cater, so my answer is likely off target. What you'll need is going to be dependent on the answer to that question, and also the manner that you work. Some people like to do one thing at a time, clean up, then go on the the next thing on their list. I tend to get everything out at once (hence my love of prep space) and clean and dispose of waste as I cook. Things move from storage to prep area to cooking area to serving area. When the food is ready there is little clean up to do, but during the process it looks like mayhem to most people. I prefer to think of the process as "magic".
