Hi Steve-long time no speak (as it were!).

To answer your questions:
a) Don't wire the old and the new in together. The new will simply discharge into the old and knacker the new ones too. Bin the old and fit new ones. 300 amp/hr should be more than sufficient for what I remember of your set up. My truck has 270 and will run a compressor fridge full time even here in the UK-though not in the winter. Personally I wouldn't bother going to the extra expence of leisure batteries per se and would just use ordinary ones. The difference is that the plates on leisure batteries are thicker and will cope better with being over discharged and over charged. Provided you look after them a standard battery will do fine. I would suggest that you ensure that the new batteries are fully and properly charged before you fit them as a solar system may not be able to do this by itself

b) There may be. Some shops have a meter which is touched across the two terminals to see what the battery discharge rate is (causes a big spark) and this will tell you whether the battery is OK or not. But if the set up isn't holding its charge then at least one of the batteries is knackered and is pulling in charge from the others. This is because there will be a potential difference between them: i.e. one will be at a lower voltage than the other(s) My advice would be to replace the lot as for example if you replace 1 or even 2 because they're wired in parallel the new one will always be better than the old one(s) and the problem will persist, at least to some extent.

c) you can mix ampere hour ratings safely. As above, the key is the voltage.

Must have a look at your blog. The last time I looked you were in Angola!

Have fun

All the best Q