I'd like to monitor, decode, and "interact" with a Modbus RTU device. Nothing complicated, I don't think. When I say interact the "linking" of meters/graphs to some of the registers etc would be really a nice to have and that seems to be inherent to your application.

Question:
Is VTscada Light the product I am looking for?

I reviewed the manual and looked at the software after installing it and could not see any serial physical layer instructions.
Can you point me in a direction or should I look else where.

Yes, VTScadaLight will get you going. The biggest restriction is that it will allow you to connect to up to 50 I/O. If you need more than that, you would have to purchase a license.

While we don't have a tutorial for specifically connecting modbus RTU, we do have several video tutorials to get you started which include connecting to a modbus TCP/IP device. Please check out the following links to get started:
https://www.trihedral.com/scada-software-video-tutorials

https://www.trihedral.com/documentation

VTScada drivers use an independent Port tag to configure the physical connection and there is a "Serial Port" tag that can be used to achieve the connection you require.

The video tutorial referenced by Dave does use TCP/IP, but you can replace the TCP/IP Port with a Serial Port and then in the driver select the appropriate Modbus type for your device, ASCII or RTU.

See the help on Serial Port tags and Modbus Drivers.