I tow a 22T with my 4.0L Crew Cab Frontier, rated at 285hp and I get by if I watch my load and drive conservatively in California ... but the 6% and greater grades and sea level to 8,000 foot elevation changes do put me in the truck lane more often then not. Consider also that once you get up to traveling at freeway speed you also have to be able to stop the thing in a moments notice or you may end up being the the first floor of a two story RV!
Keep in mind that the GVWR of your tow vehicle includes not only the empty weight of your RV but also must account for any additional load in both the tow vehicle and the RV such as driver, passengers, pets, gasoline, LP, water, hitch, clothing, food, beverages, books, you get the picture! RV tongue weight as well as GVWR are a concern ... most of us try to stay under 80% for a margin of safety. In any case a good load distributing hitch and sway control are a must!
The quick answer is that you will not regret owning the Tundra vs. the Tacoma.