If you have a 720p TV and 1080i camcorder, either the TV or the camcorder must be equipped to do the downscaling. That is, either the TV must be able to accept 1080i signals and transform them to something that it can display, or the camcorder must be able to send a 720p signal from it's 1080i internals. Read the manuals and the specs for that.
There might exist interfaces (boxes, cables and the like) that can do the conversion if the TV and the camcorder can't. Your problem isn't unique, after all. Maybe somebody else will know.
I am planning to buy a High Def camcorder, some are 720p (Sanyo) and some are 1080i (Canon, Sony, etc.). My TV is a Sanyo with 720p. My Q: will a 1080i camcorder work fine with my 720p TV or I have to buy a 720p camcorder?