are you a perminant RV'er or are you a vacation/retired RV'er? The reason I ask is because there are a few options. If you are living in the RV there are options like Belkins Tizi Equinux Tizi Wireless Tv Tuner For iPhone, iPad + iPod Touch: Amazon.co.uk: Electronics I know they sell them for people in the UK to recieve TV Broadcasts right on their iPads however I'm not sure if there are any for people in the USA. (I'm actualy in the process of starting to look for one to use in the US for Hurricane season).
The other options would work if you have a Home and go out RV'ing. You can buy a TV Tuner card for you PC and set it up so that you receive your Home TV channels anywhere on your ipad via internet. I however do not recommend this option as it is not very reliable and getting it setup can be a task in itself.
The most simple (yet not cheap) thing would be to buy a SlingBox. What you do is connect the SlingBox up at your home. The slingbox hooks into the CableBox that you have at home and then allows your iPad to watch any channels that your slingbox can get on your home TV. This means you would have full access to all TV channels you have at home and also you can watch stuff you have recorded with your DVR.