This is not a viable idea...Sorry. you would quickly exceed the weight limit with all but the smallest tanks. RV's are basically made of balsa wood, seriously. Water is the sworn enemy of RV's everywhere. One major mishap and anywhere in the zip code of the tank will be destroyed. I love fish tanks, RV's and lungfishes but IDK how you would pull this off.
A solution...I kept my lungfish in a rubbermaid tote when I moved from Florida to philly in the front seat and then in the apartment. They could care less how much room they have as long as they are able to eat and breathe. If you are pulling a fith wheel you could keep him in a container in the bed of the truck, and set up some sort of a pond when your parked.