If you buy an air awning, such as the commonly used Kampa ones for Eriba (we have one), then there is no pole attachment to the caravan and no bracket fixing - the only attachment to the caravan is through the awning rail. All awnings can leak a bit through the awning rail, but this is mostly due to capillary action through the rail connection. This can be reduced (almost stopped) by various techniques detailed elsewhere on this forum - treating the connection fabric with Fabsil or similar helps, or using Blutak in the awning rail at the end of the awning chord, which prevents water being pulled into the awning itself... I have done both those and it definitely makes seepage much, much better. Also you can buy rubber upstand profiles that go on the awning rail above the connection slot, which help redirect water running off the roof, to either end of the awning chord.
If you use a sun canopy type of awning (such as the popular Isabella Shadow) with poles that go back to the wall of the caravan to support end panels, etc, then Isabella make special brackets (Fix On mk2) that push against the wall without using screws or glue to hold them on - I have just bought some (recommended by Colin above) and will try them out next week.