Ed,
The answer is a qualified "yes" it is doable.
We have a 42 foot diesel pusher and tow a Yukon XL. The problem towing a 25 foot C Dory behind most diesel motor homes is the length. Official length limit is 65 feet on interstates, but can vary by state. Good Sam says Washington is 60' foot limit. (RV Trip wizard says 70' another site 75'?) Figure the 25, with tongue, hitch, and outboard over the end is going to be well over 30 feet--and as much as 35 feet. If you only have a 35 foot motor home (measured length, often is longer than stated length), you might possibly be under the 70 feet. Sort of a "pays your money and hope that some cop is not having a bad day..
With our RV plus the C Dory22, we are 75 feet. Many states don't seem to worry about this, and we always are taking our Yukon XL also so we can pick up the tow at any time, if we are stopped. So far, with many trips thru the South, we have not had an issue. California is very strict about the limit, and we have seen RV's stopped and measured. If this happens, you have to arrange for some vehicle to then tow the boat the rest of the time you are in that state. When we get to the last stop in AZ, we hitch the boat to the Yukon XL, and drive thru the Western States towing the 22 behind the Yukon. Also is the issue of both rest stops and camp sites if you are a lot over length. Many camp grounds cannot handle 75 feet, so occasionally you have to unhitch (or bend the train).
You probably know this, but only one version of the GMC series can be towed flat down--and you do have to remove at least one fuse. (Prior to 2014, no fuses had to be removed). The only version is the one with two speed electronic transfer and neutral transfer case. The standard 4 x 4 and all wheel drive are not towable. It was hard to find a vehicle which had the electric neutral case, unless you special order it. Towing, you put the transmission in "park", with the transfer case in neutral. Only takes a minute!
The Canyon is a bit smaller and lighter than the Yukon. You probably will be OK for short hauls. I would want electric / hydraulic brakes (which we have) on the boat trailer.
The Yukon with heavy duty tow package (same as full size 1/2 ton pickup) will tow the 25 OK, except on grades. We towed my son's 25 over the Grapevine, and the transmission overheated. (Hot day also) No harm since I run synthetic fluid--but changed it out any what. If I was hauling the 25 behind my larger truck, I would put in a second transmission cooler. The Denali with 6.2 liter engine, has that extra cooling capacity and is rated higher tow capacity.
You will know that the C Dory 25 is back there behind the Canyon... One of the reasons we have not gone back with the 25 are those issues noted above. RV has 10,000 lb tow capacity--Yukon is marginal, but "fixable with extra tranny cooler".
One of my friends who has a 25 and 38 foot pusher, just bought a Ford 2500 as his C Dory tow vehicle, rather than use the RV.
If you want to launch with the RV--consider putting a trailer hitch receiver on the front of the RV.
