I don't believe that there is a flat rate for a job like that. Assuming that you have a 30 amp feed, you probably won't be able to get access to the wire way that connects the transfer switch and breaker panel, unless they are in immediate proximity to each other. I've never done a 345, so I have no first hand knowledge here, but I have changed about a half dozen transfer switches on large MoHo's in the last few months. Access issues will dictate the time involved.
A transfer panel on a new Holiday Rambler Navigator took 4 hours and required removing the outer access door, inner bay panels and the power cord take-up reel. OTOH, changing the same item in an '06 Newell took 45 minutes because it was isolated in it's own access bay. Nice coach, BTW! But you would expect that for a mil and a half!!!
You basically have to get a good feel for the locations of the components and see if there will be wire routing issues, or whether it is a straight shot.
Sorry I couldn't 'nail' the answer for you........hope this helps a little