The TV has a label that tells you how many watts it uses (when powered by 12 volts), or the number of amps. It is amps you want - so if it doesn't specify amps, just divide the watts by 12 to get amps. Smaller is better, of course.
Next: If you multiply the amps (for example, 2 amps) by the number of hours you plan to run the TV (say 6 hours), you have the number of amp-hours your TV will consume (in this example, 12 amp-hours).
Your battery, when fully charged, contains about 75 amp-hours. Common wisdom is that you shouldn't use more than about half of this, so you have about 35 amp-hours available for use. So in this example, you are good to go with your 12 amp-hour TV. Substitute your real numbers, of course.
Naturally, if you have a solar panel, the situation is improved. But remember that this accounts solely for the energy consumed by the TV. If you run any other appliances, such as the vent fan, they are also consuming amp-hours, so battery life will be shorter.
Bill
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