Paul,
It sounds like the issues are similar on both side of the Atlantic. Pro truck drivers like yourself are likely to be very conscious of this stuff, but I was just curious how aware most RVers are over there. Hopefully, the dual language signs in Wales apply equally to low clearance warnings ;D
The extent of "advance warning" of low bridges here seems to vary by state and maybe even by county. Bridge clearances on our local country roads around this area are signposted several miles from the bridges, but I've been surprised on some country roads in other areas of the US.
The issue of road re-surfacing has been discussed here on the forum over the years, but I don't know if anyone knows for sure whether they change the signs or not. It's a scary thought, but I doubt anyone here has taken a tape measure to confirm the validity of signs.