The actual tank vent hoses in no way connect to the filler-necks; they poke through big rubber grommets in the tops of the tanks.
Like I already said, 1/2" hose will fit right into those grommets and replace the 7/16 hose that is currently in there.
Lube those old hoses with some sort of rubber-friendly slick-um; tire-lube or vegetable-oil is good.
Pull out and discard the old hoses, complete with the silly roll-over valves.
Slick up the new 1/2" hoses and poke a healthy amount through the grommets.
Route the two new hoses high and dry.
I TEEed all THREE of mine ( I have three tanks ) into one main vent hose.
I routed this up high and dry, using a barb and pipe fittings to make a downward-pointing "U" shape that prevents water getting into the vent.
A piece of screen-wire hose-clamped over the opening keeps out critters.
On writing/typing this, it came to me that one could also drill many many little insect-proof holes into a pipe-cap and this would look neater than the clamped on screen.
You are correct in that the way you guys have routed your main tank vent hoses into the filler-necks has effectively eliminated the ability of your tanks to vent; who on earth came up with such a plan ??
Plumbed thus, if those tanks are venting at all, they must be sucking air past the grommets, or through a rusted pin-hole.
Honestly, it surprises me that the engines still run.