best place to tap into 12v ignition power from the rear of the truck

If you tap into the trailer light circuit, then it would only be active when the lights were on, would it not?
You're going to have some unused slots in your fuse/relay panel... International interiors are simple enough, I'd just tap into one of those and run its own wire back. Then, you're not putting additional drain on an existing circuit, and you can set it up how you want... constant power with a switch in the dash to flip it on and off, run it off one of the empty ACC slots so it's only engaged with the key switch in the 'on' position, etc.

 

Your fuse panel... there'll be some fuse slots which aren't in use, and are put in there for different accessories which can be ordered with the truck. They still have power to them. Some provide power constantly, some only provide power with the key in the on position. You just tap into one of those unused ones and run off of that.
On the diagram, it should show which ones are run direct off the battery (those will always be powered), and which ones run off the accessory circuit (those will have power with the switch in the on position). If the diagram doesn't show you (I work primarily with KWs, and very rarely with Internationals), then a test light will... if it lights up in that empty fuse slot with the key off, then that one goes to the battery.. if it only lights up with the key on, then that's on the accessory circuit.
The quickest way is to insert a blade plug into the fuse slot, run your wire straight from that, and install a fuse into that, or you can install the clamp into the opposite side of the fuse slot (on the hot side, the clamp to hold the fuse will already be there, but they're usually not on slots which aren't in use) and make it look a bit more 'proper'.
Let me know if you're local to me, and we'll get this critter knocked out. I would expect a couple beers afterwards, though.

 

In spite of the rather northern location of this state (I'm native to MS), I love it here.
But yeah, you can run the wire down, and you should be able to get it under the mat easily enough so it's not just laid out above the floor mat, then there'll be a small hole under the mat somewhere in that sleeper where you can run it under the cab the rest of the way and to the unit you're powering. I'm just not very familiar with the layout of that particular truck, and the only truck in that family which we have where I work is a daycab.
If you got any questions along the way, I'm at your service.

 

just buy a roll of wire (auto parts store) the same gauge as the tpms signal booster uses.... just hook it up to your batteriers (with a inline fuse) and then run it to the back of the truck.....run it with zip ties along the frame rail with your tail light wiring.... DOT likes that

you will only need 1 wire as you will ground the the tpms signal booster at the back same as your tail light's

the reason you do it this way (hot wire no on/off) is because you want the tpms to always read the trailer tires just as it does the trucks..... if you don't when you turn the truck off it will lose the trailer and it won't pick them up again untill after you start driving.... maybe 10-15 miles

i have a tpms with trailer and that is the way i have it setup, it's nice when you get up and can check ALL the tires psi. before you even get out of the truck

the system will pay for it self the first time it saves you a tire.... mine has saved me 3 (nails) and 8 when TA missed aired my tires.... 80 psi. inside and 105 psi outside... saw it as soon as i got in the truck....low psi. warning for all 4 inside

 

None taken. I know wiring, but not the TPMS system, and he's clearly more familiar with it.

 

my booster is mounted on the inside of the frame rail on the drivers side at the very back
i used epoxy to stick the booster to the frame...my booster is water proof

i like your idea of the box thow.... epoxy to the frame or zip tie... remember the box MUST not have metal or it will block the signal

as to the settings i run 110 psi. in front and 105 in back and 100 on the trailer, i can set each axle for what i want, i keep my setting HIGH +15 psi over/ low -5 psi under
if the alarm goes off HIGH i can look and see if it's just one tire (somethings wrong) are all the tires running over, just the tires on that side... very warm in summer or is it just one... if it's LOW is it fast or slow (my tpms shows fast and slow) if it's slow i'll wait till i get to a ts mine will go off again at 10 psi under (if you run 110 in your steer tires do you realy want to wait until it gets down to 89 psi. to know about it..?) ... if it's fast the next place i can stop and check it.
when your driving and the tires heat up the psi will go up and in the summer the southside tires will also be hotter so the psi will be higher on that side... my tpms also shows tire air temp. thats how i know.... also handy if you have a wheel bearing going out (before you have a FIRE) or your brakes get hot.... i will get a temp alarm for 1 or 2 tires ( i think i have it set for a temp of 145 but not sure)

i've had mine for three years so when the alarm goes off i just give it a quick look and go O it's ok or deal with it.... it will take time for YOU to get to know how YOURS works on YOUR truck.... i know drivers that have had them on their trucks and less than a month later they take them off ""nothing but junk"" most of the time they don't have them set right so the alarm goes off all the time.... just stick with it, you may have to change setting a few times (+/- 1-2 psi) to get it the way YOU want it

after some time you will not even think about it being there unless it goes off.... and it's kinda fun to check it when your going down long hills (grape vine, cabbage) you can tell if any of your brakes need adjusted by looking at the wheel temp....
my tpms base has a battery so if i need to add a little i just take it with me when i air up a tire and use it for my tire guage.... i still stick check all my tires 1 time a month

hope that helps