Self contained can have different meanings. Usually, it has a water system, and power system.
A lot of slide in campers have a shower, stove, furnace, crapper,Fridge, (some fridges run on propane and electric? bed and lights, most slide In's do not have a generator. Having a generator is no biggie really, a lot have a battery to run lights, and furnace blower. If you have a backup battery you can charge one on your trail rig, and just rotate them out. I know that's what Larry Smith did when he had his. There isn't a great amount of room, but if you can handle a little cramped space it's a good choice.
This time of year is a good time to buy one. Seems they are cheaper.
Good luck