I've seen it done before using a tarp/liner in a truck bed. Biggest issue is heating. You can do a redneck hot tub which is a copper coil heated over a flame with both ends in the water, the rising heat in the coil will act as a pump, just make sure you can REMOVE either the coil or flame so that it stops heating/pumping and the coil and tube will get hot. If you only remove the 'intake' the copper can eventually melt as it can no longer transfer it's heat. Enjoy but be careful. Also recommend doing some test runs and make sure your tub is structurally sound as a large volume of water can weigh a considerable amount. If this is just a warm soak for your self you can make a barrel which will require far less water than a full tub.