I thought about that controller, but in order to get the temp compensation, you need to install the controller in the battery bay. That means you really need the remote meter to go with it, so that's $160 and you still need a solar panel. If I were going to go that route (something quick and temporary), I'd probably go with the Renogy Adventurer since it has a built-in LCD display and a temp probe on a fairly long lead (80"), and it's only $90.
I had hoped my little 5W panel would keep the chassis battery topped up, but given what I'm seeing with the set points on the Atkinson charge controller and Precision Circuits BIM, the chassis battery is getting way too low for it to even hope to keep up (about 12.4V, or 70% SOC). That means any charging solution I come up with for the chassis battery needs to make up approx. 30% SOC on a 80 AH battery, or up to 24A in a day!
That means one of two things: Get a 100W panel (and a suitable charge controller) and put it in the window of my rig when it's being stored to keep the chassis battery happy (very easy to install), or bite the bullet and permanently install a second 100W panel on the roof and replace the Atkinson charge controller with something else (maybe the Renogy 20A MPPT controller and Tracer remote meter.....only $180 for both!). The first option is really quick and easy and requires no permanent mods, the second is a bit more involved.
Of course there is option #3, and that's to pull the chassis battery disconnect to detach it from the parasitics that are pulling it down and see if my little 5W charger is able to keep up with natural discharge. I'd just have to direct-connect it to the battery vs. using the 12V
socket in the dash. Hmmmm.......