I'm going to put the instructions below, on how I solved this problem --in case others are looking for the same information.
This solution relies heavily on @Kevin Windrem 's tankrepeater instructions (which is no longer necessary with newer GX OS versions).
It is likely quite simplistic for many on this forum, but I could not find one central place for this information, so hopefully it'll be useful to some.


Pieces required:

1) SeeLevel N2K control panel from Garnet.

Note: For some reason Garnet appears to no longer offer this on their website. But I understand that one can contact them and request a "prototype" version that they'd be willing to sell.
Their current solution is an RVC device, which will not work the same.

2) GX device. In my case, I'm using a CCGX.

3) N2K "network".

Note: I do not use N2K for any other system. To build a "proper" NMEA 2000 system requires a bit of hardware and wiring commitment, and I did not want to invest in that. This solution simply connects the CCGX directly to the SeeLevel tank monitor.

3-a) Scrap ethernet/UTP cable (cat-5 or better).

3-b) 3M 37104 plug connector.
Note: Kevin's instructions recommend a 3M 37104-2165-000 FL 100 connector, but with current supply issues, I could not source that, and ended up getting a 37104-2124-000 FL 100 connector instead. This connector is for thinner wire jackets, but worked fine for me. I sourced it from mouser.com and 3M makes several of these 37104 connectors for various wire sizes. Just make sure you stick with the 37104 series (4-pin connector). I'd also get 2 of them, just in case Murphy has his way.

3-c) 1/4 watt 120 ohm resistor. This resistance value is specific to how N2K terminates the bus.

3-d) The handy VE Can-Bus terminator doodad received with your CCGX (blue RJ45 terminator), that was probably tossed in a box somewhere long ago. If you do not have that, you'll need another 120 ohm resistor (see below).


To build the "network":

1) Cut off one end of the RJ45 ethernet cable.

Note: Ethernet cables are made of 4 twisted pairs of wire.

2) Holding the RJ45 connector, with the clip behind, and staring down at it, the pinouts are numbered 1-8 (left to right). Keep the wire pairs together and take note of their individual colors (to use below).
The 4 wires we're interested in are 3-6, 7-8, and we will use them as their existing pairs.
Note: The colors will vary depending on the spec used to make the cable, but in general pin 8 is solid-brown.

3) From the cut-off end of the ethernet cable, strip back the outer cable jacket enough to allow sliding the small wires firmly into the 37104 connector, leaving their individual plastic covers intact.
The 4 wires we're interested in are 3-6, 7-8, and we'll use them as pairs. Wires 1,2,4,and 5 should be trimmed off.

4) Holding the 37104 connector, position 1 is closest to the lock-clip, and is noted on the plug.
-- Place wire 6 (V+) into position 1
-- Place wire 7 (CAN-H) into position 2
-- Place wire 8 (CAN-L) into position 3
-- Place wire 3 (V-) into position 4
Note: There is no need to introduce external voltage into the cable. If you'll be using this setup with additional N2K equipment, you'd introduce power externally, and would likely use a dedicated power cable on the bus.
-- Insert the ends of the 120ohm resistor into positions 2 and 3 respectively to connect CAN-H and CAN-L together. This will introduce a bit of bulk to the connector, and depending on which 37104 connector you went with, you may need to strip the small wire jacket to make it fit. Or you may solder it on to the wires externally.
-- With all wires properly positioned, fully inserted, not crossed or touching each other, and the resistor properly in positions 2 and 3, press the connector end closed. Note: There is a special tool used to crimp that type of connector together, but I just used a set of adjustable pliars with a flat end to press it together.

By the end of it, you'll have one cable with a regular RJ45 ethernet connector on one end, and a 37104 plug on the other end.

Note: If you did not retain or have the RJ45 terminator mentioned earlier from your CCGX, you'll need to fashion a new one. A good way to do that is to utilize the cut-off plug end of the ethernet wire, and terminate pins 7 and 8 to each other with another 120 ohm resistor (trimming back all the other wires).

To connect it all together:

1) Plug the RJ45 end of the cable to either VE.Can jack of the GX.
2) Plug the RJ45 terminator into the other VE.Can jack.
3) Connect the 37104 plug end into the SeeLevel (with the side-clip, it only fits in one way) --and with the SeeLevel, of course, properly wired to its sensors and own power, etc.
4) Reboot your GX

If the wiring is correct, you'll see the N2K device show up, and likely 3 tanks connected.


Optional:

While creating a "permanent" cable might be the more elegant/clean/non-hackey way to build this solution, I actually created a less elegant version using an extra RJ45 wall outlet plug I had lying around.
You can buy those from Mouser or even Home Depot and the like. With it, I can use just a basic ethernet cable, and then do all the termination and break-outs of the cable into the 37104 plug separately using the punch-down slots on the wall-plug (each of which is numbered).
That way, the "wall-plug" is, in essence, a custom connector/dongle for the SeeLevel, and the rest is just basic RJ45 wiring.

Additional:

If you are running a GX OS version prior to v2.70-12, the GX will "bounce" from one tank to the next to the next. Kevin's tankrepeater solution solves that problem with some custom code to better handle multi-device data. But the current/easier solution is to update the GX to the latest firmware.
Once updated, you'll see 3 discrete tank objects on the device list of the GX

You will want to go through the SeeLevel manual for the N2K to configure tank sizes, alarms, etc in order for the tank info to better reflect your actual setup.
There is a capability in the GX to twiddle some of those settings, but not all.

The built-in GX panels will now display a new panel page dedicated to tanks, and the "Mobile Overview" panel will display the new tanks on the side.
Personally, I'm not a huge fan of either of those visual options. While they are very much functional, I much prefer the look of "Flow Overview" panel. For that, I used another of Kevin's great solutions called GuiMods. It not only adds the tanks to the Flow panel, but a whole bunch of other nice visual add-ons as well.
Instructions and installation source material can be found here: https://github.com/kwindrem/GuiMods

Good luck and regards,
GM