When new (2001) the RV mechanic told me not to use the "boost" at all....something about a relay or something else having a habit of blowing out.
So...once I used it. Guess what? The solinoid clicks...but nothing happens. Guess that one time I used it...or tried to....I guess something y'gotta LISTEN to the mechanic.
No matter...if the 12V battery cannot start the motor, I do two things: If I am on shore power (like when snowabirding) and the 12V battery winds down due to not being charged for a couple/three months....I simply put a battery changer on it overnight to change it up.
Otherwise, in a hurry, jumper cables (from the toad, for instance) do the trick.