I have a 2020 3401RL.  Here is a picture from a year ago at the grey tank #2 valve looking across to tank#1 (but I think it is hidden behind the black tank vent/laundry drain pipe).  The grey tanks slightly overlap above the black tank and the tank outlets and valves are above the black tank.  From memory, there is no space to reach it from the bottom.  I think for the picture I was able to pop my phone camera lens above the black tank to get the pic.

I remeber thinking when I was taking the pic that I had read you have to drop the black tank to get at the valves and I can see why.  I think it might be possible to access fom the osb lower wall or cutting the floor, but it is probably very close to directly under the inside stair supporting wall.

A few things:

1) Original gauges are completely untrustworthy and after a few years none seemed to work properly.  I agree that Sea Level which I have installed works great.

2) On my 3401 the grey tank #1 only has the bathroom sink in it (the laundry dumps to the black tank).  The grey tank #2 has the shower and the kitchen sink draining to it.  As a result we only have <<10% of the water in tank #1 when compared to tank #2.  Installing a gate valve on the sewer outlet allows you to equalize to open both grey tank valves and equalize them allowing you to utilize the tank volume (ie you can have twice as many showers before you have to dump grey).  Because only the bathroom sink dumps to grey #1, I also though the valve might not be working as it usually had hardly any water come out when I needed to dump grey #2.  The gate valve on the sewer outlet also prevents you from getting wet when removing the sewer cap if you decided not to equalize and have kept all the drain valves closed.

3) The grey tank #2 exits out a small pipe (~1-1/2") that crosses horizontally over to the #2 tank drain which then drops down into the 3" sewer pipe.  This arrangement probably doesnt allow very good venting of the sewer pipe so every time I open the #1 valve there is a delay and limited flow at first to the sewer pipe outlet.  Once the pipe is probably vented out through the #1 tank vent, the drain volumes are good.  I've never drained both grey tanks at the same time, but next time I will with them equalized and check on my Sea Level gauges to see if they drain at the same rate or if my #1 drain valve is restricting.

hope this helps 

Thank you for your kind help and pictures.  It's apparent Redwood engineering gave no consideration to repair access when they cobbled the holding tanks and plumbing together.  Likewise the sensor system for the tanks.