The solution.. First to hold a lot of fluid you need a lot of Motor home or trailer.
OR---- a way to extend your capacity.
Last I checked camping world sold a couple of "Bladders" these are designed to go on the roof of a car or pickup,, They hold 40 or 90 gallons (About) and you fill them at the filling post and gravety transfer to your RV's tank
You can also get bags and bladders from other sources, some come with either 12 volt or 120 volt pumps so you can lay them in the BED of a pick up or a car's trunk or back seat.
That's the one I may want some day.