Our Experience:
We have a 26ft Class C and this is first year we have had a toad. It is convenient and allows more flexible travel at your destination. However, we have visited about 60% of the National Parks in the USA (plus a trip to Alaska) and a Number of National Parks in Canada without a toad. Really enjoyed using the public transportation. You get to see more because you don't have to pay attention to the road and you meet interesting people. Its much more social. I also enjoyed not having to worry about the extra vehicle plus the extra cost if you take a ferry or toll road.
My fuel mileage is 8.5 mpg without the toad. It is just a little over 7 mpg with pulling the toad. We have about 35K miles on this MH without the Toad and about 3K miles while pulling the toad so we will have a better view of fuel economy difference by the end of this year.
The downside is in places outside of a National Park where you want to go to a trail head for hiking. I don't like to leave the MH unattended in those situations so we have missed out on some opportunities.
Happy Trails