Just to be clear, you have the N300 High Gain model with the twin antennas, and not one of the other 300 Mbps, wifi-n, plug-in modules? There seems to be minor differences in software and also power.
Are you saying the screen changes to "blue lines" when you plug in the N300? Or are the "blue lines" there already and nothing changes when you plug in the N300?
What PC operating system does it have? As I recall, Tiffin used a PC-based system in the Phaeton, but its probably an old Windows, and maybe a customized version as well. The N300 requires it's own driver software for Windows or Mac OS, and that has to be installed first. They have support for WinXP, Vista, Win7, and Win8. Did the driver installation work?