My trans is due for service (done at 30k, now 58k). I have it scheduled for service (simple drain, filter change, refill). I am not planning a flush or complete fluid exchange. Truck is mostly a commuter - towing a 10k fifth wheel very occasionally.

The soonest I could get it in for service is Monday and I leave for a 400 mile R/T with my fifth wheel on Tuesday. Is this a bad idea? Should I wait until I get back?

 

The only reason I can possibly think of is if you were paranoid about the shop screwing something up causing you to have to postpone your trip. But if that's the case, you need to find another shop!

Get her serviced and go I say.

 

Why wait? Have the bands adjusted while having service done. Bands going out of adjustment will cause more damage to clutches and hub if allowed to go for too long especially when towing.

 

400 miles? Is that all, hell I do 800 in one day! I'd wait. You got enough to do getting ready, it ain't going to get any worse if you aren't having issues now.
Later you'll have plenty of time to get it done without worrying if the right parts got put in or the fluid was the right kind, or whatever.
You're call... I'd wait. If it ain't broke don't fix it, 400 miles more ain't making a hill of beans difference.