I do not have your model TT, but most of these are made very similar. I just happen to be under mine this weekend doing some work and took these 2 pics that may be helpful.
The ground wire is the little white wire in my pic coming off the lowest bottom part of the tank. It uses this as a water reference point. That white wire goes to a copper compression screw lug bolted to the frame. That copper lug at times can corrode and not make good contact.
Also the set screw that presses against the wire can get corroded to the point you cannot turn it. When this happens and you know for sure it is not making connection, well then you end up cutting the wire and replacing it. They are at most hardware stores/Lowes etc or you can use a crimp on eyelet connector. Not a lot of money. The cooper lug is $0.75 to $1.50 the crimp eyelet is like $0.15 if you have the crimp tool
To help prove out it is the ground, well first see if anything is loose. Then just take a small piece of wire and touch it to the lower ground stud on the tank, white, and then touch the other end to the next sensor stud up. On my tank it is the yellow wire. You can wrap a little wire around it (a jumper wire), it does not have to be screwed on. That should then make it read 1/3 full when you go inside and press the fresh tank sensor panel. You do not even need water in the tank to test this.
How these probes work is, as the water rises and touches each probe wire it then will indicate inside that level. You tricked it with the jumper wire.
Now if you have 1/3 level and it is now working, then something else is a not working when you add water.
But if it still is not working now you need to add 1 more jumper to check the ground reference. Take a 2nd piece of wire, wrap it around the lower white stud, (the ground,) and then go off to some part of the TT frame that is shinny metal, a bolt head or something. If everything now starts working then the copper grounding lug on the frame is having some issues.
If that is the case then unbolt the lug from the frame and sand or file the frame to make good contact and re bolt it on. If the screw is loose tighten it if you can. If the screw is so corroded it will not make contact after shining up the frame and you cannot tighten or loosen it, then a new lug one will fix that.
If this does not fix it and you want to dig deeper, let us know. If you do do you have a volt/ohm meter?
Hope this helps
PS As Kitty said the tank sensors generally works verses not work. The black tank is generally the other problem. They read false signals from toilet paper shorting them out inside the tank so it can read 2/3 full when the tank is empty. But there are tricks to cure that too.
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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