What do you mean by 'type'? Do you mean like AGM vrs wet cell? Each has its pros and cons. Whatever you get, make sure it is a true deep cycle battery. If it has any kind of 'cranking amp' rating, it is suspect. True deep cycle batteries only have an 'amp hour' rating.
Yes, rather often. For some reason, 2 6 volt batteries in series is 'better' than 2 12 volt batteries in parallel. In my coach there is actually 4 6 volt batteries, 2 series sets in parallel. The thing to watch out for is the size, particularly the height. The 6 volt ones tend to be taller than the 12 volt ones.
I use two Trojan T-105 6 volt batteries , wired in series as one 12 volt battery, in my trailer. They last a lot longer than the standard 12 volt kind because they have a lot more lead in them. They are a bit taller than the 12 volt batteries though so you need taller battery boxes also.