25ft? You drive a school bus?
I am running a 16ft (10+6) on my touchscreen with out issue. I did try 26ft (10+10+6) for just a couple hours and it did work but that was in the house and not incar near power lines.
USB seems to be a good option. I tried it and it did work partially, I was just having driver issues that made me decide to go with straight serial.
Do you plan to run serial over the rj45 (wouldn't think that was possible with only 8 wires) or do you use one of those units with the converters on each end?