Some recommend using a broomstick or other device to show you how far the slides will come out on each side. You do need to use care to not slide out into the power post, or park in such a way that opening the sewer compartment door is impossible without moving the RV.
That said, most RV parks have more than 12' of space to park in width wise. This givs you space to open 3' wide slides on both sides of a 8' wide RV, allowing up to 14' inside space. Even parks where many sites are only 12' wide, it is usually possible to find a wide site.
When looking for a new RV, look at the refrigerator. Can you get to it while the slides are in? I saw one toyhauler with 2 refrigerators. One easy to get to while slid in, the other was practically impossible to get into it without sliding out the kitchen slide, and that is requirement to use the stove.
Don't think you will need into the refrigerator while the slides are in? Ever stopped in a small town to re-stock the RV? Yes it is nice to put the groceries away, especially the ice cream. And get to some foods while in a rest area, or parked along a scenic overlook, where putting out the slides might not work so well.
I have seen several RV's with dual propane powered refrigerators. This might be very handy when full timing, or even for alarger group, say a family of 5 or 8 taking out a RV, with perhaps a family of tenters, and all using the RV refrigerator to store everything.
MOre recently, I have seen rear outside kitchens with a small 120 volt refrigerator accessable only from the outside kitchen. Easy to open hatch that also acts as a roof for the kitchen, there is a place to plug in a TV along with a cable socket. While I am sure that most of the RV owners are not able to run a inverter or generator while traveling, the refrigerator should stay cool or even cold for a few hours while driving from the home hookups to a campground, and back. A blue ice pack will extend the time you can leave the trailer unplugged, while the refrigerator contents stay at 41F or less.
My next RV, I will be looking for a full length wall slide out on the drivers side, perhaps a outside kitchen, and other features not found on the last new RV I bought in 1996.
Fred.