Trying to maintain a charge by having a small solar panel on the inside of the is a good idea, but it more than likely will not meet your expectations. The small solar panels (like any solar panel) need to be angled up toward the sun to capture as much of the sun's rays as possible just to keep the batteries topped off. By setting it in the window it has already lost most of the suns rays. Secondly, the windows are usually tinted so that will reduce the solar rays even more. For small systems you need to put the panel/controller as close to the batteries as possible.
You would probably get better performance if you were to place the solar panel in the bathroom shower skylight, which lets a LOT of sunlight in. The issue there is the distance between the panel/controller and the batteries.
If you decide to go with a small system that will only be used to keep a charge in your batteries, make sure it has a small solar charge controller. Some of the inexpensive units say to connect the panels directly to the batteries, you really do not want those.. as most of them do not even have a blocking diode in them and when the sun goes down the panel actually discharges the battery.
I am sure some of our members that have this setup will let you know what they use or what to stay away from.
Just my thoughts,
2013 Jayco Eagle 284BHS
250Watt Grape Solar Panel, MorningStar MPPT 60 Charge Controller
1500 Watt Ramsond PSI, 2 Trojan T145 Batteries (260Ah)
2 - AirSight Wireless IP Cameras (used as rear view cameras)
EnGenius WI-FI extender, D-Link wireless (n) modem
MagicJack Internet Phone
2012 Ford F150XLT, EcoBoost w/3.73
157" Wheel base, HD Towing Package
Our Solar Album http://www.jaycoowners.com/album.php?albumid=329