We made those moves. From a 33' gasser to a 40' DP, and now to a 42' DP. And, we're very happy we did.
When we had our 2004 40' Allegro Bus we started to think about a 42' tag axle coach. During the last year or so of owning it I paid attention to campsites as far as having enough room for a larger coach. I found that 95% of the time we could easily fit a 42' coach in the same site as a 40' coach. The couple of times that we couldn't, all I had to do was park the Jeep sideways in front of the Bus rather than end to end. So, no real drawbacks in that area. As a side note, going to a 45' coach would change those odds and I'd say we'd probably lose about 1/3 of our sites. However, this all will depend on what kind of places you go to. Many nicer RV parks can easily handle a 45' coach while the smaller state parks basically eliminate you at 35'. We tend to stay at middle-of-the road RV parks most of the time so that's what my findings were based on.
Other pros and cons:
A 40' coach has a single axle. With quad slides and the latest bells and whistles, solid surface countertops, solid wood cabinets, and full ceramic tile you start eating into your cargo carrying capacity. My 2004 Bus had over 5,000 lbs CCC but it was a 2 slide and wasn't as loaded up as the newer ones. Many of the current 40' quads have a little over 3,000 lbs CCC and the rear axle weight needs to be watched. Our 2007 42' (actually 42.5') Bus has everything, including residential 21 cu ft fridge, washer/dryer, dishwasher, etc. With the tag axle we have in excess of 9,000 lbs CCC and there's no way I could ever haul that much unless I start hauling Lucille Ball's rock collection.
The tag axle gives you an awesome ride and takes stability, especially in cross winds and 18 wheeler bow wakes, to a whole new level. It's like riding on rails.
Plus, the IFS and automatic tag dump make this thing just as maneuverable as our 40' Bus.
You also get a couple of more feet of interior room over a 40 footer.
One possible drawback is basement storage space. Whenever you add a tag axle, it eats up close to 4' of basement storage space because you can't put any cargo storage compartments in that area. So, in theory, if you want the same basement storage space you need to go another 4' longer, not 2'. Seeing as how they don't make 44' coaches you either have to go down to a 42-43' coach (usually they are all 42-1/2 feet give or take so a coach that's advertised as 43' probably isn't any longer than one that's advertised as 42') or up to a 45' coach.
In our case we really liked the Allegro Bus 42QRP floor plan and features. Going up to a 45' Zephyr didn't appeal to us. While it was a nice coach, the decor and floorplan of the Zephyr just didn't appeal to us. Plus it was another $100 grand which put it out of our league and we weren't ready for a 45' coach yet so that worked out really well. I had our 40' basement packed pretty tight and was concerned about loss of storage space. When I looked at the 42' Newmar and Country Coach offerings I wasn't impressed with the smaller basement storage. However, we were pleasantly surprised with the way Tiffin laid out the 42' Allegro Bus. They only offered the 42' Bus with hydronic heat, which is what we wanted anyway. By doing this they reallocated the space normally used for the hot water heater and dual propane furnaces as storage and designed the basement area around the hydrohot system. This gave me the same amount of basement storage as my 40' Bus, if not more, so I had no loss of basement storage space. Whichever brand you choose, be sure to look at the basement storage space to see if it'll work for you.
The only other drawback I see to an "over 40" coach is a legal issue. 40' coaches can go anywhere but there are a few restrictions to anything longer. If you want to go to Zion Nat'l Park you can't go through the Mount Carmel Tunnel, you have to go around and approach the park from the west. Also, California has a few roads, mostly in the Sierras, that do not allow anything over 40' on. Check out the California Bus and RV Restrictions Map and Website
for details on where these are.
Other than the possible loss of basement space and a handful of over-40 road limitations I really don't see any problems with a 42-43' coach. If you want more details and pictures of a 42' Bus, check out a review I made of a 42' coach at This Link