This may be a necessary step depending on how many layers of existing roof there are.
How much to shingle a trailer roof.
How much does a shingle roof cost.
The benefit is it costs less to install and may look prettier than say a steel roof which may last 40 years but has a higher upfront cost of installation.
If you need to have an existing roof removed prior to installation expect to pay an additional 100 150 per square to have the existing shingles removed.
An experienced handyman will be able to install a rubber or metal roof while a roofing contractor should be consulted for a composition shingle roof where installation prices vary from 675 2 500 for a single wide to 1 300 5 000 for a double wide depending on size of unit location and shingle quality.