I installed SeeLevel gauges on my fifth wheel - definitely not the same as your Greyhawk but the installation principles will likely be the same. The gauge kits come with excellent instructions which are somewhat complex but well explained.
SeeLevel gauges use a self-adhesive sensor applied to the outside of each tank. One wire can connect several sensors and carry the digital coded signals to the monitor panel. Each sensor must have the same ground plane as each other sensor and the monitor panel. You can use your existing probe wiring if you want, but be careful that the grounds are all the same; do not just ground to the frame, run a dedicated ground wire to the trailer DC ground buss. I gained access to the tanks from underneath by removing the coroplast bottom.
My trailer had a single panel with the probe sensor lights and switches for the pump, hot water electric and hot water heater LPG. If you plan to put the SeeLevel monitor panel in the same place, you'll may to figure out how to handle the switches (since on my trailer they were part of the tank monitoring panel). What I did was to cut off the un-needed probe monitoring lights, then closed up the remaining part of the old panel that had the water heater and pump switches and attached the SeeLevel gauges above it.
Here's a link to a post about my install: SeeLevel
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch