My 2006 T-139 has the same small gray and black tanks that you have. According to my dealer, there is no way the size can be increased because of the overall design of the unit. The 2008 units have much larger capacity tanks because of redesign, but we're stuck. We take along a portable waste water tank made of molded plastic that we can fill and roll (it's got wheels on it) to the dump site. It's a 25 gallon unit we purchased from Camping World at a clearance price because they were discontinuing this particular brand. We paid $60 for it. The current models they offer are more expensive. Look around on the internet and see what you can find. Hope this helps!