I have solar panels attached to my Yakima racks. Thule is very similar.
I used the threaded track nuts (the nuts that hold the towers to the track) to hold aluminum square tubes cut to the appropriate length and then bolted the solar panel frames to the aluminum tubes. Quite strong and low profile. Aluminum "L" channel would probably work as well. I can still use the towers with cross bars to carry items such as kayaks. The tower height puts the crossbars comfortably above the solar panels. See the photos in my gallery.
I assume that the batteries will be connected together into a single battery bank, in which case you only need one battery monitor. I use the Victron BMV-712
I am not as familiar with Lithium Ion batteries as I am with lead acid but it is my understanding that Lithium Ion batteries typically have built in battery management systems.
I hope this is somewhat helpful,
Hopefully someone more knowledgeable, such as Rando, will comment.
Edited by ckent323, 15 February 2020 - 07:08 PM.