You'll need to do some math - since we can only set the start date of the trip you'll have to figure out when you need to start in order to make it to the desired stop at the desired time. The easiest way is to set an arbitrary start date to determine how long it will take to get to the destination, and then reset the start date accordingly. RVTW keeps track of the dates so you can easily work out the start date.
For multi-month trips I usually set an arbitrary start date, and then plan out the trip. Once the trip is pretty well planned we then determine if any stops need to be "set in stone" - if so then we need to adjust the start date. I've found that it can help to
convert dates to a Julian number, add or subtract as needed to find the required start date, then convert back to calendar dates. We used this method for our trip coming up this year: we have 2 stops that are set in stone: the FMCA Northwest Rally in Oregon, and the FMCA International Rally in Nebraska. All the other stops are based on getting to those 2 stops on time.
When the fine folks at RVTW add this functionality to the program our lives will be easier!