If I were faced with that job, I'd likely pull the tanks to know what I need. Not going to be fun for sure. To change, I would start by pulling the toilet, and then the closet flange and pipe leading into the tank. Find the vent pipe leading to that tank, and do what ever is necessary to remove it from the top of the tank. Same story on the grey. Remove the inlet and vent from the top of the tank. From there I can't be much help. Too many different possibilities.
New tanks generally come with the outlets installed, but no inlets. You'll need to transfer the dimensions for those from the old tanks and install your own.
Though certainly possible, maybe not the best DIY project for most people. Access to the various tank manf. catalogs might be pretty important, and then, something that's just pretty close may have to be modified to work.....
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake