typical gen install charges are around $5k minimum. (google search for onan generator)
usually at least $3500 for the gen. Most on the lot RV's are not gen ready. You can control it wireless with the Onan EC-30W controller, but its also around $550. Then you need the safety enclosure, wiring and the transfer switch..
I'd call your prefered RV dealer service dept and ask them what they would charge to install a gen set, new or used.
I am in the process of installing a 7kw onan into my not gen ready FW.
On the other hand, I don't think you would get $5k extra for your RV with the gen installed.
is your current FW 30 or 50 amp service? what will u be looking at?
