I've completed the work and decided to provide the answer for others wanting to know the same thing. We all know probes suck, but some of us would still like something, even if a bit inaccurate.
As to removing the underbelly. It is really just corrigated plastic with thin foil insulation. I lowered it from the axels on back. A few screws and it came right down. The sensors for the black and gray tanks are on the dump side, so I could have just down that section to work on them. Reinstalling it is a bit more difficult. Lining up the screws to the holes on the cross members was a bit difficult, so I measured from the main beams inward to the hole and them held up the liner and was able to get the screws in. Doing a couple this way and the rest line up.
I was leaning towards iSeries (https://tankedge.com/products.html
) but every retailer was backordered. I contacted the company and was told they had stock and it was likely because of Covid and retailers not wanting to stock shelves. A bit suspicious, I held off and checked out the SeeLevel. The tank was around 3" or 4" on one end and went to 6.5" on the other. Crosswards it sloped to the drain and it was close to the main metal frame. There was wording embossed in the tank so it wasn't a very smooth surface. SeeLevel reviewers said it worked best on a flat surface, non-irregular tanks and more than 2" from metel. I decided against those for now. The iSeries looked like a perfect solution, but want to make sure they stick around for support/parts before I spend $300. With all of that, for now I went with the Miracle Probes.
Removing the old probes was easy. I found a tip online where you remove the nuts, put a large washer on and one nut towards the end of the threads. Wedge a open end wrench under the washer and put something behind it to pry. Doing that, every one just popped right out. The Miracle Probes went right in. The reviews were all good, but time will really tell how they work for me.
The black and gray tanks are 30 gallons so I decided to measure when the lights came on. I get it, all 19RBs may not be the same, this is just what mine was drilled in as. The 1/3 light came on when the tank was 1/6 full (5 gallons). The 2/3 light came on when the tank was 1/2 full (15 gallons). Then the full came on at 70% full or 21 gallons. I put in 30 gallons total and it was completely full. The water level was coming up the drain line from the toilet.
The two lights that were off completely were the result of two different issues. As other posts on the forum have shared, the wires may need to be reset. The nuts was all tight but one wire was loose from the ring connector as it wasn't properly crimped. I redid the end and the lights came on. The fresh water was more difficult and I traced it back to the fuse panel. It wasn't a fuse, but a loose wire at that location. By the way, to work on the fresh water probes you can leave the underbelly on. You can pull down the liner from the main beams and reach in to them. The sensors are right in front of the fresh water drain on the entry door side.
Hope these tips are helpful for anybody else looking to work on the sensors.