Not sure what a goose-neck adapter is....unless you are talking about a clear adapter that goes on the end of waste tank drain line and it has a hose connection that allows you to run water backup thru drain line ??
Anyway......2006 Jayco is a used trailer. Therefore the level indicators are suspect because it doesn't take much to foul the 'sensors' up. TP, dried stuff, slime, just being wet........will cause them to indicate.
You can drive yourself silly trying to keep the sensors clean and indicating correctly only to have them foul up next trip out.
Surprised only one in just black tank----usually at least 2 and grey tank also.
You can use the GEO Method (water softener/dish soap/water mixture) to clean up tank & sensors to slosh around while in transit
You can use bags of ice down toilet to scrub tank while in transit
You can purchase numerous products to clean/deodorize/sanitize/micro enzyme treat tanks
You can pour buckets of hot soapy water down toilet and repeatedly dump tanks
You can back flush gallons of water thru tanks
You can drive yourself insane.......
OR you can enjoy camping and dump rinse tanks when needed and forget about the level indicators
Toilet 'burps' when flushing-----time to dump black tank
Sink/shower drain slows down/doesn't drain as fast as normal-----time to dump grey tank
On-board water pump goes '''burrrrrrrrrup'----should have refilled fresh water tank sooner cause you have run out of fresh water
Couple of trips you will have an idea how long YOU can go before needing to dump/refill
I haven't looked at our tank level indicators in over 6 yrs. Don't care/don't need.
Same goes for that 'battery condition' indicator too.
It will always show FULL if connected to shore power and various levels when not on shore power.....but what exactly is 2 lights??
I check my batteries via hydrometer and handheld multimeter
Go camping.....enjoy and dump when tanks are full