Like the OP, I was never able to import the GPX file output from RVTW into the RV890. The phone -=> RV890 path is very fragile in terms of communications and content. I don't think the RVTW GPX file contained with some header sections that the 890 expects.
At any rate, I was able to write a little app to convert the RVTW exported Excel spreadsheet into a GPX file the 890 could read.
The help file is located at:
If you can't read it, go to findwaterfrontbar.com, ignore anything there, then go to the help file.
The password is: RVTW_To_GPX.