My '77 Excella 500 31' did not have a fresh water drain either. You have to use the on board fresh water pump to pump it out. What a pain so I carried 5 gallon buckets of fresh water.
'01 2500hd ext. cab, 8.1 litre gas, 5 sp. Allison auto
3.73 rear end
Mag-Hytec rear diff cover
Amsoil Dual by-pass oil filtration system
Amsoil synthetics all around
265 watt AM Solar, Inc. system