The simple answer is yes, It will handle it.
What you'll need to do is connect the + of the new battery to the + of the existing one, and the - of the new one to the - of the existing one (or direct to earth provided you have a good earth). This basically means however that you have a twin battery system, so it may be as well to ensure the existing battery is in good shape, otherwise you are connecting a new battery with an old one, and thats never a great thing.
As for wiring, make sure the fuses that you put into each circuit are a little weaker than the strength of the wire. This means if the + wire shorts out to the body (the earth), then the fuse will blow before the wire melts or goes on fire. So, figuring out the size of the fuses depends on the size of the wire. And put the fuses close to the battery, they only blow if a short occurs after them in the circuit. so, there is no point putting them near the end of the circuit.
if the max load of any of your items is about 5 or 6 amps, then use a wire for abut 10 amps, and a 6 amp fuse. use a much heavier wire from the battery to the old battery, capable of handling 100 amps or so - if a battery goes dead, or runs down, then the alternator kicks in, you can see 100 amps on that wire (in theory, 130amps, but that's not really going to happen in practice). You might also like to put in an isolator on that circuit so during a weekends camping, the fridge does'nt flatten both your batteries, and you can still start the engine.
Hope this helps!
as to solar, that's a bigger question! do you mean to keep it charged when parked for a day? a month?
Merv.