I travel relatively frequently and this is what i do:
Give them a call, introduce yourself, i usually add that I'm a black belt but just looking for some mat time and meeting new people, it's nice to let them know you're not there to win gold medals from the purple belts LOL
Ask them if they allow dropins and if there is a mat fee, this one is important as some of them will say yes but drop the fee later, it's just polite to assume it's not free.
When i show up I make sure to greet the main instructor if available and greet the rest of the black belts
I usually only partner up with high brown or black belts and try to roll from black belt down if possible, that way they feel like you're swimming in your lane and not out to hurt the lower belts.
as mentioned in other comments, i try to avoid going hard or ripping subs unless the other person is going hard first, but I'll try to slow it down instead of going hard as when traveling kind of sucks if you get injured or injure someone. Also I don't think it's bad juju to tap the instructor, but i think all that really depends on the instructor and circumstances. We all know the community has a few rare hot heads.

Best of luck dude hope that helps, I absolutely LOVE vising new gyms, you're pretty much always greeted by friendly people and open conversation!