[Targan]

Broadly speaking, slavers would fall into 2 varieties, those who are catching and transporting slaves to sell to others, and those who are catching slaves for their own use or the use of their patrons.

The latter type of slavers may effectively bear official or quasi-official "letters of marque" from a government or military command authorising them to capture and indenture fugitives, outlaws, bandits, maurauders, escaped POWs etc. Keeping such captives well-fed and healthy may be a low priority, especially if the governing body receiving such slaves intends to work them to death anyway. Some slavers may not have official sanction or support from a government or military command but will receive bounties in cash or kind for appropriate sorts of captives to be handed over.

The former type of slavers would definitely find it in their best interests, commercially speaking, to keep their captives healthy. I don't know where your current campaign is set, BT, but the problem with being a slaver and slave trader in a place like Twilight War Poland is that the whole region is a hodge-podge of overlapping and shifting jurisdictions and depending on the jurisdiction slavery will be anything from openly embraced to frowned upon to highly illegal. Indeed, areas under martial law may punish slavers with death or effective slavery themselves.

Moving chain gangs of slaves through areas where slavery is illegal would be frought with danger for slavers. Hefty bribes placed in the right hands might mitigate the risk somewhat, as would travelling away from major roads and moving at night.

Just a few random thoughts.