Welcome to the forum.
That 900 lbs on the
title is usually the dry
weight before options are added and the trailer is loaded for camping.
The first thing you should do is go to this site:
http://lakeshoreimages.com/spreadsheets/Weight.xls
This spreadsheet is based, for the most part, on actual trailer weights taken at various FGRV gatherings as folks arrived, so they reflect the weights of trailers loaded to travel.
I think there is only one
Trillium 1300 listed, but you can get an idea from the other 13 footers;
Scamp,
Boler,
Casita, etc. What you can expect.
The next thing you want to do is check your owners manual to see what your Outback is rated to tow. According to Edmund's the '08 Outback is rated for 2700 lbs. Actually I tow a Weiscraft
Little Joe with a 2014 Outback 2.5i CVT. On the 2014 that 2700 lb tow rating is WITH trailer
brakes, so you will likely need those if the trailer doesn't already have them, as well a a brake controller in the tow vehicle (TV). Also, current Outbacks and likely your '08 are limited to a 200 lb tongue weight. Your owners manual is your best guide.
In addition to a new spare, you may want to check the age of the
tires on the ground. Trailer
tires are better changed by age than by wear, as over the years they can dry rot and, despite little tread wear, blow out. Go here to find how to find and read the date code on your
tires:
What is a tire date code and where do I find it? | TireBuyer.com
Regarding the
battery, how old is it and more importantly how long has it been sitting around. If it has been unused for a number of seasons, it would be best to change it. Get a deep cycle
battery. A group 27 Deep Cycle should be good for 85 to 100 amp hours. Some folks are going to AGM batteries, which are great if the
battery is inside the trailer, but if it's on the tongue, save your money.
To answer the
solar question, you/we need more information. Specifically what is your expected usage. You can find some good information on 12 volt systems, maintenance and usage here:
http://www.marxrv.com/12volt/12volt.htm
As regards usage, one of the best ways to reduce usage is to replace incandescent bulbs in the trailer with LED.
Solar will be necessary if you plan to stay at places without electricity. (Note: my
Little Joe is solar.) Normally, if you have AC available, the converter charges the battery, but on a trailer that old, you would do better with some sort of smart charger, as the older converters were known to overcharge batteries.
Good luck with your preparations. There are lots of folks here willing to help, when we get more information.