We transitioned from a 3 bed 23' HTT to a 32' Class C MH with 2 slides with 2 bunks on the bedroom slide.
We keep both at a storage yard, but the cost is the same. Our annual vehicle registration increased from around $100 for the HTT to $500 for the MH. Although that cost drops each year; for 2013, I think the MH was about $430.
Cost of insurance went up, gas mileage went down a few miles per gallon so we spend more on gas each trip.
One of the down sides for us was getting used to not having a small vehicle when we got to our destination. Most times that is not an issue, but if we go to conferences or something like that with the MH, then we are usually renting a car. Hopefully before the end of the year, we will be purchasing a 4 down towable car so we can bring a car along with us. That too will be an added cost - the cost to setup the MH & the toad for towing.
For us, the added cost is well worth it. We use or MH a lot more than we used our HTT.
-Michael