Black water is not usually the issue. Most people can easily go 5 day on a tank that size. Grey water on the other hand, people have issues with filling very quickly. Many places state it is against their policies to dump the grey water onto the ground.
Personally, I have not heard of anyone putting in a larger tank. Generally speaking it is not a pleasant job. With an enclosed underbelly, it is a huge undertaking, and new brackets will need to be made.
There are a lot of different tank sizes and configurations. I would take a look on Amazon to get a base idea of tank options. https://www.amazon.com/RecPro-Gallon...633661&sr=8-38
Personally, I would invest in a good quality portable waste tank with good wheels (maybe a wheel upgrade) that you can pull behind your TV.
You can also use a portable macerator pump to fill/empty the tank