extra battery in tahoe, do i have it right

Batteries shouldn't be paralleled when idle (not in use). But if they are, join the two matched batteries together. Take the +12V from one, and the ground from the other. That helps ensure symmetrical loading and charging.    But when one battery deteriorates, the downward spiral that destroys both batteries begin (ie, when non-matched batteries - not the same condition, age, temperature, characterisics etc).

just so im understadning... so i need to runs a ground from battery to battery aswell as the positive..?? this that what yoru saying?

Connect + to + and - to - The +12V is taken from the + of the one battery. The gnd connections (to body & engine etc) are take from the - of the other battery.

so i have the + to +  and on the second battery i have it grounded to chassi.. Is that ok or do i need to run a neg lead from my stock battery to extra battery?

You connect it as I said. The two interconnects (+ to + & - to -) should be matched (same size & length = same resistance). There should only be one connection to +12V vehicle wiring and one to gen (from the opposite -ve (not on the same battery) from where the +12V is taken. Note that this is ONLY intended for permanently paralleled batteries, and it doesn't prevent earlier failure - it merely removes unbalanced loading and charging that results from different electrical paths (but not from the different charge/discharge characteristics of each battery). The unreliability of a parallel battery is the sum of each unreliability. IE - for matched batteries, the unreliability doubles. (The unreliability being for cell collapse issues.) If you only connect the batteries when charging, and (say) with ignition on, then the above isn't as critical (assuming each battery has time to recharge).