Some TVs now have a signal strength indicator. Look real good through the menus - sometimes they are buried pretty deep. If you find one, rotate the antenna slowly as there is usually a delay in the indication.
Broadcast stations are frequently (but not always) located in the same location; to get the first one so you can orient the antenna, do the auto scan and stop it after the first channel is indicated. The TV receiver will usually allow you to abort the auto-scan and receive the first signal you get. Use this as a "beacon".
The next method (and probably the best) is get out a map and compass and orient the antenna toward the closest major city.
The typical RV antenna is directional broad side to the long axis of the antenna; and toward the side away from the support mast.