Usually I tour in lycra bib shorts, sometimes loose mtb shorts with padded undershorts. I have toured in street clothes, though those have tended to be gentle, shortish affairs.
Wearing lycra bibs isn't a problem as far as washing is concerned. They can be washed out each night. If you wring them dry in a towel they'll be dry in the morning in most conditions. If they aren't, having a spare pair weighs little and doesn't take up much room.
Obviously, if your saddle suits you it's perfectly possible to tour without cycling-specific clothing. But the longer the tour, the more time you'd have to manage in the event that you did get saddlesores or similar, so a long tour might not be the best time to experiment.