We have a 2008 Mobile Suites 38RLSB and are fortunate to have access to both our tanks (black and grey with little effort). I ordered and rec'd the Horst Probes last week. I ordered them just for the black and grey tanks as our fresh water tank probes are just fine. In fact, our grey tank sensors were worse than our black tank sensors for some reason. I figured all the money I spent on sensor cleaners thus far would make up for the little bit of $$ these new sensors cost.

Here is my install story...

I first went to the hardware store to purchase some wire and jumpers in case I needed them...I then moved our sliding storage tray out of the way and began studying my next adventure. Oh, I also emptied the tanks as well and notified my wife not to use anything that has water tied to it.

I then started with the grey tank first which is the bottom tank for our rig. I was able to locate all 3 sensors and the ground. I started with the top sensor (full sensor). Disconnected the wire and began to try and loosen the factory installed probe. I could not get the sensor to turn or budge so then I went to plan B which encompasses drilling new holes into the tanks to install the new sensors. As long as I didn't screw this step up I figured this was a better way to go because you can always move the wires back to their original locations if for some reason the new sensors fail. Granted the original sensors don't work well but at least they are there for use if needed.

I removed the sensor wire from the top sensor (full) on the grey tank. Luckily, on our rig, there was quite a lot of the original extra wire so I could install the new sensor where it was handy to drill. I decided to move it around the corner of the tank so the rear of the sensor (where the wire is attached to) faced the front of the rig. Drilled the hole, placed a little marine grade sealant on the probe seal (this is something I did for peace of mind), and pushed the new probe in the new hole. It fits in the 3/8 hole pretty snug. Hint: Do not push the probe into the hole by pressing on the screw where the wire is connected to. You may actually push the probe out of the fitting and cause extra work for yourself or potentially push it into the tank...yikes...

I tightened up the first nut to create the seal, put the wire on and tightened up the second nut and done. here is a picture..
You can actually see the factory installed probe location around the corner to the right of the new probe.

For the second or 1/2 full sensor, negotiating the drill into a desireable location was the hardest part. Once I got that done the rest was easy. here is a picture of the second / 1/2 full sensor...

From there I spent about an hour trying to figure out how to get a 3/8 hole towards the bottom of the grey tank. I finally gave up on that until I could look for a creative way to drill (e.g. flexible or 90 degree drill bit may work but I did not have those with me). In addition, you really don't care if the tank is 1/3rd full, you are more interested if the tank is getting close to being full. 

I also decided to forego installing the ground sensor. The reason being is that if you eliminate the ability for the probes to connect via sludge and stuff on the side of the tank, the original ground should work just fine. Also, the ground wire for each tank is extremely hard to get to. If needed, I would cut the wire and install a jumper and move the probe to more easily accessed location.

Onto the black tank which is situated above our grey tank. I found this tank much easier to install the probes. The hardest part was working around the large flexible vent that was directly in front of the probes. I decided to move the 1/3 (bottom) and 1/2 (middle) sensor around the corner and the top sensor right next to the factory installed top sensor. Luckily the original probes had enough wire to reach my new probe location and no jumpers were necessary. This picture is looking up towards the black tank.  . Once again, I did not install a new ground sensor even though it would be pretty easy on this black tank. I may do so at a later date.

Went into the rig and checked the panel and all looks good. Time will tell if they work when stuff starts to build up, but, from the description, they should work just fine....and as I said before, since I haven't removed the old sensors I can always go back to them if needed.

On our Mobile Suites ....Red = Top sensor or Full wire, Green = middle sensor or 1/2 full wire, Yellow = bottom sensor or 1/3 full wire. Wiring colors were consistent for both tanks.

I'll keep you posted on how they work.