I have a fifth wheel that I have always used four Trojans in. I do alot of dry camping and find they come in useful. When I bought my current trailer I moved two of the Trojans from my old trailer that were already about three years old. When they finally wore out and needed replacement the oldest two were shot and the newer ones were reduced in output, but not entirely shot. I just replaced all of them at once. The older batteries made it 11 years and the newer over 8 years.
In theory, the combination of the two 6 volts and the two 12 volts could work and be done, but considering the limited life left in the 12 volts batteries I would just bite the bullet and go with all 6volt. I should say this. Based on having used both 12 volt and 6 volt batteries in the same (previous) trailer, I found that the 6 volt batteries seemed to charge faster and last longer. When using the 12 volt batteries, despite having approximately the same total amp storage as the six volt batteries, the 12 volt always seemed to run down faster.