A motorhome is a motor vehicle, so it needs a title, tags, and insurance while in use. It may be possible to suspend part of the vehicle insurance while it is parked for extended, but the tags (registration) are issued on an annual basis. Mosts states require that liability insurance be maintained on any vehicle that has an active registration tag, since they have to assume it may be driven at any time. Depending on the state the motorhome is titled in, there may be a substantial tax applied to the value as well. Several states have persobnal property taxes on vehicles, often substantial ones.
Since a trailer is not a motorized vehicle, no liability insurance is required, and the lack of a drive train (engine, transmission, etc) reduces the initial cost as well as ongoing maintenance. Registration fees are usually much less as well, and the lower cost reduces any applicable taxes.