Toilet paper: Scotts or Angle soft
A few Dogbones (30-50 and 50-30 amp)
30 and 50 amp plugs (a couple of each)
Assorted 12 volt light bulbs and fuses.
Potable water hoses,, And Hose washers.
ONE or Two only RV-Water filters,, (The inline type) I like the larger diameter filters.
Though I do not normally use them "Black tank products" I suggest carrying only the waste digester types, NOTHING with Formaldehyde in it. (You do not wish to preserve the ...er.. stuff).
Picnic supplies (This is fairly common line of products in such a store)
On the RV specific stuff like Dog Bones, Hoses, filters and waste digester only carry a SMALL inventory at first. Plugs for example If he carries ONE 50 and one 30 amp.. Then when that one sells, odds are he will have time to re-stock before he sells another.