We've had a 25'9" Class C for 4.5 years. We just got it set up for towing this summer (despite purchasing our toad a few years ago). We tow a tiny '96 Toyota RAV4 (2-door, manual, AWD, about 2,600lbs). Our RV has the Chevy chassis and we can tell the vehicle is back there, but it's not a problem. It didn't even seem to affect gas mileage on our first trip (not a lot of data, but the calcs were in the same range we're used to seeing, depending on speed, headwind, etc.). On a particularly steep uphill in Colorado recently, we were definitely traveling slowly and maxed out, but otherwise we really haven't seen any performance reduction from it. And I'm not certain we've been on that stretch of road without the toad, so I didn't have a baseline to compare to.
For our normal roadtrip style travel (5,000 miles in 2.5-3 weeks) we probably won't drag the toad. We're always on the move and the RV is short enough not to be a problem in getting where we want to go.
For a trip to one area (i.e. we spent a week in Jackson/Tetons this summer) we'll probably take the toad. While there are times I love having the RV everywhere we go (e.g. getting caught in a downpour while hiking in Olympic NP and having the RV with towels and dry clothes at the trailhead), there are also conveniences to having a separate vehicle to explore in.