Originally Posted by ddezurik
Assume non matching batts.
That's the key here. Batteries of differing size, age and type should not be wired together. One problem as mentioned already is that one battery will be drawn down faster and deeper. That will be the truck battery as its not made to be discharged by more than 5% or so before the alternator recharges it. You will be replacing it sooner than later.
The better option is just to buy a battery similar to your current deep cycle or marine and either install it or swap the two when the TT battery is discharged.