79 GMC 18.5 ft roof strong enough to stand on?

Depends on who built the roof (not General Motors, rather some RV manufacturer) and how.

Mine is Filon supported a solid foam core, it adequately spreads foot size loads. Others may be waterproof fabric over plywood spanning rafters, and how it handles loads depends on plywood weight and rafter spacing. If you don't know the construction details, it is best to use sheets of plywood big enough to span any likely rafter spacing, which could be as much as two feet.

Good idea with the planks and plywood.Thanks I will try that.

Hi--in my experience--NO. Better to put some planks across that will support your weight on the outside sidewalls. If you can't find planks long enough, at least span you weight by spreading plywood, about 1/2 to 3/4", on the roof and step on this.