If "custom" comes from Shasta, it probably means nothing.
If "Custom" comes from Ford, it is a trim level. Probably one level above "Base" which means it got sun visors and probably at least two seats, and maybe a chrome or painted grill (instead of black). Heater might have been standard equipment. But that still doesn't mean much because Shasta/Coachmen likely ordered it with an upfitter's option package which included some things that differentiate Custom and might have dropped others.
Equivalents today would be XL and XLT, although there is an XLT Premium option and several "delete" options to downgrade from XL to a plainer work van.
Tom Test
Itasca Spirit 29B