The same thing has been happening to me. I have an 8 gb micro sd and I still have 80+ mgb lefts on my phone. And I keep deleting, and deleting, and it KEEPS saying I have no room. And when I only have 60 texts, it says I have no more text space. wth?!?!?!
I spent some time today making more space on my wife's LG Vortex. We had moved all the apps to the SDcard as we could but there still wasn't enough space. I was able to free some more space up by uninstalling the updates to various VZW bloatware apps through the manage applications menu in settings.
I did at the same time incur the nasty "Incorrect Package Signature File" error that prevents installing or updating any apps. A battery pull, downgrading the market and clearing the market cache fixed this after a few goes.
Another option would be to root the Vortex and remove the bloatware. I meet be doing this for my wife in the next couple days. If I get the time and everything goes well I'll let you guys know how much space we are able to free up.
Some extra advice on low memory on the low RAM phones. Besides moving apps to the SD card download a cache cleaner app from the market, keep your text messages cleared or use SMS Backup and Restore to back up your texts (does NOT do pictures) to the SD card to empty the phone texts. SMS B & R allows you to keep the backup on the SD card and view that backup so no need to restore! Also, check you email programs and select the menu button within the email app to view your folders. If you are using only Gmail, you will need to log on to Gmail on a computer to do the file deletions. I helped someone who had 1700 plus emails in the trash folder! There was no 'select all' so this was going to be a long process to delete them. It might of been a Citrix... But the same should apply to the Vortex. To clear things up every now and then, perform a factory reset to clear up any bogus stuff that never released memory. According to LG website, the Vortex has 150 MB of internal memory. The Samsung Continuum has 384, Charge 512, and the Bionic 1 GB. That is the core issue.