Depends what engine the LPG is fitted to. As we're talking about Land Rovers, I assume you're talking about a V8.
On a V8, MPG will range from about 14-18mpg on LPG.
With a TDi, 27-28 mpg is achievable
With a TD5, 24-26 mpg
A TDCi is much the same as a TDi with regard to mpg
And you won't really want to tow with any of the other diesels, you'll be lacking in power.