You can call around to some local body shops...they should be somewhat familiar with the install. Of course, the RV shop will probably have done a lot more of them, which may make a difference to you. It really depends on just how much of a difference there is in the price.
I don't know if the RV Service Shop will really be upset that you didn't get the base-plate from their Parts Store. Work is money, and Money is King!!:B Unless they are really crazy busy, I'm sure they will be more than happy to do the install....for the right price!!