I actually have my eyes on this one.
More than just maintenance, may be enough to go off the grid for a week.
The fact that this can be stored inside, and then placed where you need it in the sunlight, makes it a winner for me.
https://www.amazon.com/dp/B07FH85FW9/?coliid=ICCARXX3XG3VW&colid=1XSB9F8J8R1OK&psc=1&ref_=lv_ov_lig_dp_it

My thoughts are to put a 2-pin connector, linked in line to a solar controller to the Aux battery.
https://www.amazon.com/dp/B07BHN1VJH/?coliid=I16M8DM1WXM4PS&colid=WQ7V2JH6L9T1&psc=1&ref_=lv_ov_lig_dp_it

Solar Controller 
https://www.amazon.com/Little-Story-Regulator-Controller-12V-24V/dp/B07RMLJTNX/ref=sr_1_19?keywords=solar+controller&qid=1563316886&s=electronics&sr=1-19

And for less than $250 I'm off the grid.

Jack

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Jim. Felder
Sent: Tuesday, July 16, 2019 4:54 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Product comments: solar battery maintainer

Since my Vanagon doesn't get the use it should sometimes, the battery can
go flat in a few weeks. I have been using this inexpensive solar maintainer
all summer at it has worked out great. It's not a charger, but it will
maintain whatever level the battery holds when the unit is plugged in. From
Amazon. There are a zillion choices but this is pretty close to the one I
bought.

<
https://www.amazon.com/dp/B07QJ8KZS9/ref=sspa_dk_detail_5?psc=1&pd_rd_i=B07QJ8KZS9&pd_rd_w=63j0Z&pf_rd_p=8a8f3917-7900-4ce8-ad90-adf0d53c0985&pd_rd_wg=yLuph&pf_rd_r=SHQ7MYYNCH43GESNJRS8&pd_rd_r=385b3a56-a80b-11e9-932e-a50aff64ee55
>

Jim