If you don't want a portable genny then what the mechanic offered is the next option. Typically you don't see onboard gennys until you are at 27' and up but doesn't mean it cannot be done.
You are looking at the cost of the genny, cost of a small water tank, cost for thru hull for the genny, plus installation of it all.
1997 2400 SCR
5.7 Vortec / Bravo 2