Installing a grey water tank is one of those activities that many vintage owners undertake at some point, just to get up to modern standards. If you do the majority of your camping with full hookups, then there is no need to have one, and you can probably get by with a blue-boy when you need to.
For your trailer, you would want the tank behind, and as close to, the axle as possible. There are many threads about installing grey water tanks, how to suspend them, what size, what supplier, etc., just do a search. Be warned, that it is no simple undertaking. If you haven't replaced your axle, then you might want to try that first.