If the Solar PV panel doesn't have to be portable then something like this might be useful;

30W Monocrystaline Panel + 5A Solar MPPT charge regulator

The Duracell DPP-600HD Powerpack 600 Jump Starter & Emergency Power Source with Radio might be also worthwhile allowing you to charge the Trent IMP 880 battery pack during overcast days.

http://www.amazon.com/Duracell-DPP-600HD..._bxgy_ol_text_b

A DC-DC voltage regulator would also be useful if you wanted to step down the voltage from 12V to 9V, 6V, 5V, 4.5V etc.

I use a similar setup for a solar powered hi-fi system. (can also be used for also powering various battery chargers i.e. such as Ansmann Powerline 5 charger or Wireless router if required.

http://forums.equipped.org/ubbthreads.php?ubb=showflat&Number=175102