Towed my FunFinder with an '08 Hemi Commander... IMHO, the Hemi is a marvelous towing engine, but, like all gas engines, they like to rev... I wasn't aware that Chrysler put the Tow/Haul system in a manual transmission truck, but, what the heck
My Commander had the "auto stick" automatic transmission...most of the time; switch the Tow/Haul on, put it in drive and let'er rip. No need to manually shift to a lower gear except on long steep grades or "technical" slow speed grades (lots of twists and turns). Otherwise, just let Tow/Haul take care of the shifting of the automatic transmission, 95% of the time you should let the very well designed Tow/Haul + automatic transmission do its thing. Tow/Haul will change the shift points to keep the engine revving (it's supposed to rev) and it (Chrysler's Tow/Haul) does, indeed, lock out the overdrive to keep from toasting the overdrive lock up. Acceleration? You won't have it. Gas engines are free revving, but, add a bunch of weight and your rear end's gearing (probably 3.73) and you lose the "umph" until the engine gets up to that magical 3800 to 4200 rpm range where it is doing it's best work. RVing isn't a race. Let the engine build up gradually...you won't be the first off the line at the light, ever.
Most good sized gas engines will develop peak horsepower and torque in the 3800 to 4400 rpm range, the Hemi doesn't red-line until between 5600 and 6000 RPM (depending on application). Don't be afraid of it, that's where the engine likes to be and believe it or not, it is much more efficient at that range. Fuel thirsty, yes, but, efficiency and peak performance are found at those rpms. Hauling up a steep grade? Downshift (either manual or with the "stick shift" automatic until the engine's tach reads between 3900 and 4200 (for the Hemi). It will pull all day long and never complain, when ever I encountered those situations, I'd aim for the 4000 mark and keep it there until I'd crested the hill or was done with the twisting tediousness of a winding steep grade. Easier on the automatic transmission that way.
A diesel, on the other hand is just the opposite. Doesn't like to rev...develops huge amounts of torque at relatively low rpms (my Cummins Turbo Diesel's max output (800+ ft. lbs. of torque) happens at between 2000 and 2200 rpm. That's why they (diesels) make such good "tow" engines. They don't have to get up to 3800+ rpms to get to their peak output (my RAM red-lines at 3500 rpm, it would blow up before your Hemi is doing its best work) Their downside? Diesel engine and the cost of the diesel option ($7000+ on the RAM over the Hemi version), diesel fuel and it's cost, weight (the 6 cylinder diesel in my RAM weighs 976 lbs more than the 8 cylinder Hemi), noise and rougher idle. If all you were going to do is tow with a vehicle; get a diesel or if you need tow a great deal of weight get a diesel. If you tow infrequently and use the vehicle for around town, get a gasser; it is up to the task of occasional towing and is cheaper in the short term.
MPG? You are doing just about "par for the course". You are towing a square box that has a frontal area of 8' x 10' in round numbers. That's a really big square sail that you are dragging through the air pressure created at 55-65 miles per hour. Weight really doesn't figure prominently in towing. A short lightweight trailer might get 1 or 2 mpg better than a heavier longer trailer given the same frontal area
. It is what it is and your mileage is pretty much dead on for what the majority of RVers get. Whenever I plan a trip I allow for 10 mpg as an average. A lot of uphill or strong headwind? Figure 8-9 mpg...a lot of level flat land or good tail wind, 11-12 mpg. A good average for some hills, both up and down and level areas with some high speed (58-63 mph) between 9 and 10. Trip average? Most will find that they will be in the 9.5 to 10.5 range. You will find some that "claim" 13-15 mpg, some even higher... I always take those with a grain of salt (fish stories comes to mind...
), but, I suppose there are some that achieve that; most don't. I got 8.5 to 11.5 towing my 6,000 lb FunFinder with the Hemi and I get 9 to 12 with the diesel towing a 12,000 lb. Cougar. That's just RVing. If you are looking for fuel-economy, RVing is not a good hobby!