I'm working on a project with the RG-11 and needed to use the serial output. I played around with it today and got the serial output working.
(etc...)
I'm curious about your choice of the RG-11 for a rain gauge, and about the project in general. The RG-11, by the admission of the mfr, and in anecdotal evidence of my own and others here, is not a very good rain gauge if you are looking for accurate rain fall readings across differing intensities of rain.
Also, as you have seen, the thing is a relative power hog, especially compared to a reed switch/tipping bucket.
I'd be happy to continue on a conversation with you about this, either here on WxForum, or in email, or in comments on the Freaklab site. There are lots of off-the-shelf stations that already seem to do most of what you need, and in fact, I've developed something very similar based around Davis Instruments stations, my own interface box, and a cellular router. See
http://www.weatherelement.com/condorlookout. This unit runs on solar power, updates once a minute to the server, and runs on about 7 AH/day at 12 VDC on a solar power system.