We bought a 26.5 RLS in 2012 and, by itself, I believe we would be fine with a tow-ready half ton, because this FW only drops my 3/4 GMC D/A combo crew-cab by one inch when we hitch it up and put the jacks up. But we have a diesel due to the fact that we tow a 19' Crestliner behind the FW when going to the lake to fish and recreate. I'm not sure that a half ton would handle all that weight when towing, but our diesel does it just fine. Since you are talking about a larger FW, I would suggest that you go with a 3.4 ton, gas or diesel will probably not be that big a deal. The diesel will last, probably, longer than the gas engine and I would have to disagree with a previous poster, I don't believe that maintenance is much more expensive than a gasser, if you don't have major problems with the engine or the transmission. Price is another consideration, as diesels are quite a bit more expensive than the gassers similarly equipped, but the resale value is also quite a bit higher down the road so you might not be out too much money overall. With three slides, that will be a bit heavier FW than mine so I agree with the other posters, you probably should look pretty hard at a 3/4 ton over a 1/2 ton.
2012 GMC 2500HD D/A Crewcab
2012 Jayco Eagle SuperLite HT 26.5 RLS
2009 Crestliner Superhawk 1900
2013 Polaris Sportsman 550 EXP
2008 Polaris Sportsman 700 X2