Volvo information
Volvo Trucks USA is the Volvo website. Trucks talks about the tractors. Comfort, safety, fuel mileage, etc.
http://www.hhrvresource.com/VolvoModelLine.htm is general info about Volvo Tractors.
http://www.jackdanmayer.com/converting_your_hdt_to_a_motorhome.htm has Volvo model numbering system info.
http://www.youtube.com/watch?v=ub-5pw3Zg4M Interior tour of Volvo 670 tractor.

Why a semi tractor (HDT)
HDT = Heavy Duty Truck.
MDT = Medium Duty Truck. These are between pickups and semi tractors.
LDT = Light duty truck. These are pickup trucks, including duallys, up to one-ton models.
http://www.jackdanmayer.com/choosing_between_an_mdt_and_an_hdt.htm is about MDT vs HDT. Their site has a lot more info as well.
http://www.advrider.com/forums/showthread.php?t=701488 is a good thread about why an HDT.
http://www.jaycoowners.com/archive/index.php/t-5541.html has a discussion of HDT vs pickups.
http://www.rvnetwork.com/index.php?showtopic=101994 makes several good points in favor of HDTs.

http://www.irv2.com/forums/f21/ is the IRV2.com HDT forum.
http://www.rvnetwork.com/index.php?showforum=32 is the Escapees HDT forum.

Tractors for sale
Central Carolina Trucks searches for and resells Volvo Trucks. Also does conversions for 5th wheel RV.
Truck Paper.com lists trucks for sale.
RV Network.com has tractors and RVs for sale.
http://www.hhrvresource.com/MemberTrucksForSale.htm lists member trucks for sale.
http://www.commercialtrucktrader.com/Volvo-Tractor-For-Sale/search-results?make=VOLVO|2314540&category=Tractor|2008002lists Volvos for sale.  commercialtrucktrader.com/ lists trucks for sale.
http://www.generaltrucksales.com/ has used trucks.
http://trucker.com/Truck/Search/Results/Tractor/volvo lists Volvos for sale.

Tractor conversion info
http://rvhaulers.ca/ does RV mods.
http://rvhaulers.ca/build-rv-hauler/ details the modification of one tractor to a RV hauler. They have a video as well.
http://www.dmbruss.com/zRedRover/RR_HDTConversion.htm is Dale and Mark Bruss' conversion of Red Rover. Also a lot of other info, including why an HDT, about their lifestyle, safety, etc.
http://www.jackdanmayer.com/converting_your_hdt_to_a_motorhome.htm has conversion info and Volvo model numbering system info.
http://lscx.net/ shows a good conversion process.
http://www.hdtrvwhitelightning.com/ has a lot of conversion info.

Tractor photos
http://hhrvresource.com/PhotoPages/TruckPhotos.htm has lots of tractor photos.
http://tinypic.com/view.html?pic=z1ysw is a Volvo with a Jeep on it.
http://www.truckconversion.net/photopost/index.php has a variety of photos. Look for HDT threads.