The coverage on a trailer is collision and comprehensive (non-collision damage or loss) and is generally inexpensive (depends on value of the trailer). For vehicle liability purposes, it is covered by the tow vehicle while underway, so no separate vehicle liability is needed..
Liability for injuries that may occur in/around the RV when parked is a different coverage, sometimes called "vacation liability". It covers you if somebody trips on your entry step or walks into an awning support and cuts their head. It happens. If you have homeowner or renters insurance, the personal liability part of that usually extends to wherever you are visiting, e.g a hotel room or a park. Ask your agent about accidents while you are temporarily staying on a campsite. If this sort of personal liability is not provided by your homeowner or renter insurance, you may want to get an RV-specific policy that has vacation liability coverage.