881 can arrive at a remote gate or a fixed gate; I am about 50/50 on 6 flights this year. For the most part it doesn't matter if you are in F. At a remote gate you will be the first off the plane to a waiting bus used only for FC pasengers, so you will be in the terminal close to the stairs leading up to the connecting security area, then down the escalator to the left and into the RCC. My sense is though, with this change 881 will get a fixed gate, and likely be used for the onward flight to BKK. All connecting flights at NRT require a plane change, that is, you leave the aircraft, clear security, then board another aircraft for your conencting flight (which may or may not be the same aircraft you arrived on).

I do not know if the FC section of the NRT RCC has separate showers. I'm guessing yes, but am not sure.

You may not have enough time for a shower, but you will not know until you arrive at NRT. Just head to the RCC and see if you can get a shower done by 6:00 PM. (If you depart from a remote gate I would ask at the RCC if you have enough time.)

I would change to 883 if you can. I tried for my flight this week but was unable to ahead of time due to some complications with the way my ticket was issued. I plan to try and stand-by for 883 as it shows an empty FC cabin currently while 881 is nearly sold out for ORD-NRT. I was going to check a larger bag but now have to scale back to a roll-a-board for this trip as I will stand-by for BOS-ORD as well.