Some thinking out loud here....
In some ways, I like the idea of adding secondary tanks rather than replacing the existing ones. But I do see some real challenges in doing that:
First, both add-on tanks would need to be vented. If it's not possible to run a vent line from the new tank to the existing vent stack, you'd have to install separate vent stacks from the new tank to the roof of the trailer. Depending on location, that could be fairly easy or a real beast of a job. Adding a new, separate vent stack also includes punching through the roof.
Second, a secondary gray water tank is much less of a problem in terms of drainage, but a secondary black water tank could be very difficult. If it is not plumbed properly, "solids" could get into the secondary tank and not have a reliable way to be drained away by gravity.
A lot of these issues would depend on the available space under the trailer and the room to route drain lines and vent stacks. The physical layout of some rigs might lend themselves to a fairly easy install of secondary tanks, while others may prove very difficult if not impossible.
Gray water is very easy to deal with via the "little blue totes." We just picked up a 25 gallon one this summer, and it has made our life much easier when we dry camp.
Black water is the problem. The "little blue totes" are not really recommended for black water removal (said so right in the instructions that came with ours.) It can be done, but there is a high potential for a spill, and a black water spill in your campsite is not fun to deal with.
So, based on all this thinking out loud, my thoughts would be:
a. Get a blue tote for gray water and leave the gray water tank in the TT alone.
b. It may well be possible to either swap out the black water tank for a bigger one, or to properly add a secondary tank that will drain correctly. But it will require that someone knowledgeable and trustworthy look at the specific TT and give a realistic idea of which plan would work and how much it would cost.
Or, adopt some strategies for making your black water tank last for a longer period of time.