Most tank heaters are a pad that the tank sits on or is pasted low on the sides. The degree of difficulty depends on the accessibility to the tank, but it's often difficult. MH tanks are typically well buried. And then you need to find an adequate power source. It's usually 12vdc, but some units can use 120v as well.
This article (it's part of a product ad) helps understand the choices in selecting a heater.
https://www.ultraheat.com/selecting-a-tank-heater
I personally would not want to pay RV shop rates (typically around $120) for a labor intensive job. Especially with the typical shops dismal quality standards.