Just my opinion, I'm sure other may disagree. If the the old batteries are in good condition (have them checked by a good battery dealer), then there should be no problem. I added a second identical battery battery to my trailer a year apart and there was no problems with either battery. I also had to to do the same with my truck, and again no problems. In both cases I had the original batteries check and they were both in excellent condition before adding the new ones. If the old ones are in less than excellent condition, then I would replace all of them.
2005 F-250 XLT 4X4 V-10
2006 Wildcat 31QBH