Assuming your propane tank is full, if the WH problem isn't that you just need to be more patient to wait until the air purges from the lines from the tank on the front to the WH then it may be the thermocouple (that's an easy fix). Also be sure to check all the push-on connectors on the control module. I had a similar problem keeping ignition (even though mine was spark-ignition) very early on... although the connections looked good, I had to replace some of them until it was reliable and then never again had a problem in 8 years.
The blank tank issue is probably because it was used improperly by the former owner. During our new 5er PDI, the tech had a well rehearsed spiel... "There are only three things that go in the black tank: human waste, water (and lots of it) and TP". He said that if we ever brought it in for draining problems, one of their shop techs would power-flush it into a 5 gallon bucket one at a time, then pour each bucket into the shop toilet through a strainer. If he found anything that was not one of those 3 then it would not be a warranty issue and would be charged at a highly punitive rate.
The tank sensors are just metal studs that go through the tank wall (if you find one of Herk's many pictures of the system, you see only one wire connected to each). The indicator works simply by showing you which ones are leaking minute amounts of current (normally supposed to be just through water, but other moist things can fool it). If the sensors get fouled with TP then thats when you can get false readings. A swizzle stick
may help with that.
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam