I'd start with looking at how you want to use it. The smaller the trailer the less "house like" I guess is how I would say it. A wet bathroom or a couch/dinette that has to be converted into the bed was not for us. We looked at many styles of trailers but due to my experience with semi's once you get to a certain size, I believe the 5th wheel is the way to go. We had a list of features that we wanted and to find them we ended up with a larger trailer. Melony calls it our 2nd house as it's basically a 2 bedroom, bath and a half house on wheels. We take 1 or 2 trips a year and the rest of the time it's parked at our home campground. The larger the trailer the larger the tow vehicle and large pickups are expensive. For the price of the 5er and the 1 ton dually I could have bought a decent used class C or A. A class A may be in our future due to some medical issues but for now the 5th wheel suits us just fine.
As for mileage, diesel gets better mileage than gas but you have a higher initial investment. Several things effect mpg. Since most trailers are boxes they have a very high drag coefficient so that tends to override effects of weight or length. I get around 10-11 mpg with my diesel pickup pulling a 12,000#, 36', 5th wheel trailer. A Sprinter based Class B diesel will probably do a little better but others can give you better numbers. A gas class c I believe is around 8 mpg
When towing a "bumper pull" trailer the weight is carried behind the rear axle and can lead to sway issues and steering issues due to "unloading" weight from the steer axle. A good WDH (weight distribution hitch) with sway control can correct this. Since the 5th wheel carries the weight over the rear axle these issues are negated. It's why they build semi's as 5th wheels. The 5th wheels on my semi's have been movable to load distribution and clearance issues with drop frame trailers. I can feel it in how the truck drives when the 5th wheel is slide to the rear (more like a bumper pull).
Travel trailers tend to not have generators where the Class A,B, and C's tend to have them as standard features. Many people uses small Honda (pure sine wave) generator as they are "quiet" and portable. Solar and extra batteries is also an option but I have yet to research solar. Others use solar and can tell you more. Since I need AC due to medical issues, solar does not really work for me.
Go to some shows and look at features available. Make a list of features you want and those you do not want. Make sure you are comfortable moving about. Pretend you are using the toilet and the shower and see if you can "take care of business". Personally, I dislike small bathrooms/showers and the toilet in the closet. I did not want to have to run to the shower house in the middle of the night. Also look at tank sizes if you wish to boondock. A small fresh water (FW) tank is not a big deal if you are always in the CG, but boon docking tank size limits you stay.
The 1st tank to fill for us is the kitchen sink (gray #1) from washing dishes, followed shortly after by the shower (gray #2, I like my showers
) around day 3 and the black (toilet) tanks (black #1 and #2) last around day 7. We have 3 people so a single person would last longer. But something to think about. Also can you reach the dump handles or are the tucked under the trailer. Drain lines are often under.