One thing that you should get first is a towing guide from each of the big three. Then you can educate yourself on towing needs, things like GVWR and axle weight ratings. Cargo capacity of various vehicles.
Then take what is written with a grain of salt, because Ford, GM and Dodge test their lightest trucks and give weights that a base model can carry. If you select a upgraded truck (most people who are not buying a fleet truck for a business pick one with upgrades) it might have 100 - 150 pounds of options. So the published tow ratings will be reduced by passenger weight, options, and anything you might want to carry such as a generator or firewood.
Fifth wheels can be very nose heavy, and yet some models can be nose light - depends on the manufacture, and where they place the fresh water tank. In the 90's it was popular to place the tanks all under the bathroom, and this causes the hitch weight to go way up on some models.
Some manufactures such as Glendale RV put the fresh water tanks between the axles, leading to several models with less than 2000 pound hitch weight. But that is rare, and Glendale did not survive the high gas prices of 2004 - 2008.
Trouble is that a manufacture can "Publish" a certain 'dry weight' and it can be way off from reality, yet the manufacture will not get in trouble with anyone, and can keep giving out un-realistic information. RIVA will not sanction them, or no government agency will stand up to them and say you are selling trailers without the proper weights published.
In the 80's Road Queen and other manufactures sold RV's that the empty curb weight was nearly as high as the GVWR. Road Queen even sold some models that where heavier than the GVWR when shipped from the factory, leaving no room to bring along camping gear, or passengers! With improved GVWR of vans in the 90's and 2000 model year, this is a thing of the past with motorhomes, and Cargo Carrying Capacity now must be published in motorhomes and must be "at least close to correct"
You asked about gas or diesel. That really depends on the yearly mileage. Do you plan on towing more than about 5,000 miles a year? You might want a diesel for that kind of yearly mileage, but the gas engine will be less expensive to buy and maintain at the 5,000 mile per year level. Around 15,000 miles a year towing then the diesel might be less expensive to maintain and buy fuel for it. However it will still be much more expensive to buy.
Some are willing to pay a lot more to buy a diesel and enjoy them. Personally my gas motorhome is great, will pass a DP on a steep mountain grade, and I like the ability to do a oil change for about $50. Not the 35 quarts that my buddy has to use in his. . . My motorhome has a 17,000 GVWR and was 16,500 pounds while on a 2 week trip with full water and fuel. His has a 28,000 GVWR, and I have no idea what the actual weight is. However it is higher than mine, and does not go up the mountains any faster than mine. I drove both, and both are fun to drive, handle winds equally well, and road imperfections are better in the Country Coach. But I don't know about filling that 120 gallon diesel tank! He seems to get around 9 mpg, 1 better than my gas motorhome. But I bought mine for 20% of his MSRP.
Fred.
Money can't buy happiness but somehow it's more comfortable to cry in a
Porsche or Country Coach!
If there's a WILL, I want to be in it!
I havn't been everywhere, but it's on my list.
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