You have to ask yourself how much trouble do you want to go through, how much capacity do you want, and how much do you want to change the interior/exterior look of your trailer.
A good place to start is with the grey water tanks available from Vintage Trailer Supply. These might mount inside your frame and either stay within the frame rails, or at least not hang much below them. You could put a grey water tank below the subfloor, and a black tank above the subfloor, but this might require a lot of modifications to the bathroom.
There are many suppliers of tanks that make tanks that will hang below your bellypan dramatically. There are also options to get custom tanks made, but these will be expensive.