Many tunnels, especially in the eastern US, do not permit campers and may limit/prohibit self-propelled units because they restrict any carrying of propane in the tunnels. In some cases you can "seal and certificate" by having the gas shut off, inspected, and seals applied to prevent turning on without breaking the seals, but the procedure is too complex for more than exceptional cases.

"Coming unhooked" is extremely rare with common US equipment as long as the towed unit stays right side up (and simple inspections will show when it's otherwise possible). I don't know of a tunnel where that's the reason cited by "Camper/RV advisors" in route information for a prohibition on towed units, but do believe an "enforcer" might use it as an excuse since it's simpler than a real explanation.

US and UK standards and practices are very different for hitches and other trailering devices, especially for "civilian uses." I've found that it takes significant research to comment "across-borders" without looking like an idiot, so in a few of my better moments I've tried to avoid the more obvious blunders. (I'm aware that success has been "variable," so there's no need to remind me of previous posts.)

John