Is it
acanthosis nigricans?
Mine come and go depending on how tight I am with IR control and weight loss.
I've had it on neck, elbows, knees, ankle, inner thigh, under breasts, armpits etc.
Right now mainly inner thigh, ankle and elbows are the worst.
If you think it is that -- get a check up and if it is that, don't worry to much about vacation. Have fun with it and just take it one thing at a time. AN clears up on its own once the reason causing it is taken care of.
A.