Not all campers built years ago were fully selfcontained. Some just had drains for the gray water that went on the ground not to a holding tank. Most if equiped with a toilet had a black tank but not always a grey water tank. You maybe able to add a hose type fitting to the end of the drain and use a hose to run it into a blue porta tank. Before doing much with the furnace check the whole gas system for leaks the proced with caution.
'03 F350 DRW 6.0 auto
2003 A990S Arctic Fox