You can monkey around with the circuits if you want, and eventually you will figure out all the design considerations, or you can go to a marine/rv center and see what they have, talk with their techs, and get exactly what you need. They may even build it for you.

Deep cycle batteries are not recommended for jumpstarting other vehicles, nor for starting your own vehicle. If you are going to use the car's existing chargins system to charge the secondary battery (highly recommended), then use a good quality automotive battery. Deep cycle batteries do not like a constant charge current either, so an auto battery is preferable for a solar panel charging system as well, unless you want to spend a lot of bucks for a regulating system.

If I were building a backup battery system for a vehicle, I would most likely wire the batteries in parallel using 12 or 10 gauge welding wire (properly fused), and have a cut-out switch so that I could go from one battery to the other but not both at the same time. This way I won't overload the alternator trying to charge two batteries at once, thus frying the alternator. I might build a little timer signalling device to remind me when one battery has charged and I can switch to charging the other. Then when I need to use battery power, I can go from one battery to the other as well. As far as the solar panel goes, just wire it with an accessory plug (cigarette lighter) and plug it into the jack on the dash to charge whichever battery needs it. The panel will never put out enough juice to have to worry about overloading the current. Even a big panel will only make about 15 to 20 watts peak.

I spent a good chunk of my life building and maintaining battery powered circuits, and the cheap way is to use car batteries and make as few changes to the existing circuitry as possible. Where I've had to use deep cycle systems, the cost has been significantly more.