Ratbat -
I use thin wire twist ties (like what come on your loaf of bread) through either the road wheels or the under hull (depending on where it wont be seen). Take a look at how kid's toys are packaged nowadays with all that wire passed through a carboard backing holding the parts in place.
Just get your tank situated, drill some 1/16" holes through the diorama base and pass the wire throught. The wire's twist will be on the bottom of the base - you have have to chisel out a recess to hide it so the base sits still. Make sure you use small wire and if you scavenge wire fron the kitched strip off the plastic coating as it will be too thick and noticeable.
After everything is mounted I usually go back and add a tuck of grass, some rubble, or whatever to help hide the wire. A couple small chunks of brick doesn't look out of place under the tank and can break the wire's line enough so that it's not noticed.