IF the TV has a power brick you have options. 
Radio shack sells cords with assorted power tips (two items) take the TV in with you or take the power brick in and select the tip.  You an get a cord with tinned leads you "hard wire" or one with an "Accessory Plug" (They do not call it ciggy plug any more) Most of those plugs by the way are... Basically junk.. but that's another matter.
IF you have the wall plate with the 12 volt outlet.. It might work. i'd loosen the plate enough to slip a bungee cord behind it to hold the plug in the socket.
MAKE SURE the tip is properly installed (Proper polarity) or damage will occure.
NOTE:  Some people worry about the regulation on a 12 volt outlet in a motor home, Voltage can go as high as 15-16 volts.. They fear, and some claim, this has damaged their televisions.
(MY "12volt" tv is actually a 14 volt TV, so I'm not so worried, However I have not the proper plug so I use the brick)
Option 2 is to use a small inverter.. I favor TRUE SINE WAVE. the 12 volt outlet in that plate (See comment about bungee cord) can support 300-400 watts IF YOU ARE LUCKY.  Smaller TSW's are hard to find. (Though they are out there).
Why do I prefer TSW.. MSW cause radio frequency interference and I'm a radio operator... They can upset digital TV too.. Depending on the make and model of everything (inverter, TV, Sat receiver, and so on).
They can also work perfectly but it's a bit like Las Vegas.. You lay your money down and you take your chances.