While you are trailering the trailer is covered by your tow vehicle's liability coverage. It still applies if the trailer breaks loose and damages something. That is covered under the property damage portion of you liability coverage. If the trailer injures someone while towing (including breakaway), then it comes under the bodily injury coverage of your tow vehicle laibility. It is no different than if the tow vehicle itself does the damage/injury.
Vacation liability has nothing to do with towing accidents. Vacation liability covers you if someone falls down the trailer steps or splits their skull bumping into a slideout corner or awning arm while the RV is parked. If you already have home owners or renters insurance, you may have coverage for off-premisies liability (personal liability) and may not need vacation liability. Check with your home/renters insurance agent.
How much is enough? Most claims are either fairly small (several thousand for hospital emergency room visit) or huge ($50k-100k & up). But huge claims generally only happen if you have insurance - lawsuits are not common if there are no "deep pockets" to go after. No insurance reduces the likelihood of a large claim. You will have to figure the risks yourself.
