Fifth wheel mileage question

My last 6 trips or so have been traveling the exact same 350 mile route going to visit her aging mother. I average between 8 and 13 MPG a trip. The only difference is the wind. This is with a gasser however

Weight matters. Getting an extra two and half tons moving in stop and go traffic requires a lot of energy or fuel. Pulling an extra 5,000# up a hill also takes a lot of juice. Weight also matters when it comes to stopping your rig as retarding all that extra momentum will create a lot of heat energy which needs to be replaced as fuel.

I always strive to always keep my weight down as much as possible by loading lightly and leaving unneeded stuff at home. Lighter rigs are easier on your truck's drive line components and will result in fuel savings. Whether the savings are that important to you is something you will have to decide.

I'm of the opinion that blaming aerodynamics as the major source of poor fuel economy is a rationalization for towing heavy.

2012 F-350 CCSB SRW 6.7 3.31 axle. 24' box 6500 lb., 27' fifth wheel 9400 lb., & 37' fifth wheel 13,500 lb all have averaged around 11 mpg. I believe that aerodynamis is most significant effect on towing mileage.

This is our 1st long trip with our new 5er, 14k gvwr. We've gone from so. NV to Reno-Elko-Boise-Lakeview-Medford so far. Tow vehicle is a 2017 Ram with Cummins 6.7.
We've driven from hot desert to high mountains and everything in between from 2700ft elevation to 7300 foot passes.
MPG has been anywhere from a high of 12.2 to 9.3.

We usually averaged between 11-12 mpg towing our 12,000 lb. 5th wheel. We averaged around 10-11 mpg towing our 15,000 lb. toyhauler. Same truck, same height, same width, heavier trailer.

IMO, I'd be more concerned about length rather than weight. Length will affect your RV site choices.

Aerodynamics are the biggest issue but particularly in hilly/mountainous terrain, weight will have an impact.

GVWR is a good surrogate for what to expect the trailer to weigh. They don't put bigger beefier axles on unless they need them. You won't find a big difference in weight between trailers with similar GVWR. About the only exception I can think of would be if you compare a toy hauler to a standard 5er but don't take any toys. The toy haulers are designed assuming a fairly significant load in the garage.

I agree with the person who said heavier FWs often equate to quality. I also agree with the two that said, don't sweat about the mileage; pick the floor plan that works best for you. On that, I have to add that after four travel trailers and knowing what we liked and didn't when we started looking for an FW, we had every salesperson bring the slides in so we could see what that felt like as far as walking around when we stopped at a rest stop or had to spend the night in it that way at an overnight stop. That was an eye-opener. In our FW, we can access all kitchen appliances, the commode, and can sleep in it, all with the slides in. Good luck in your decision.

I would not be overly concerned with the weight/height but really concerned with the floor plan I wanted. Most fivers run 13 feet and a few inches to top of a/c's regardless.
Now your mpg will run 9.3 to 10.5 depending on the terrain and more importantly your speed. I had a F-250 diesel pulling a heavy 28 foot Sunnybrook fiver and ran 10.5 fairly consistently. Our 40 foot motor home with a cummins got 9.5 to 10.5. Now with the setup in my sig I get 9.3 the majority of the time. I run 65ish on interstates and 59ish on primary roads.

I will stress again though, go with the floor plan that you want or you will be sorry a few months, years down the road.

My experience is weight makes very little difference in MPG. My 5er is about 12,000 pounds. I often tow my 3500 pound boat tandem. MPG is about the same either way. And it doesn’t really matter if I’m towing relatively flat (not much of that in Colorado) or through the mountains.

Rig in my sig. 50,000 miles. Average 9.5. Been as high as 12.1 as low as 8. Sometimes the mountains are steeper, sometimes the wind is at your back.