The best bang for buck mod you can make is the programmer and tune from 5 Star Tuning (
https://5startuning.com/product/7015fs/)
It was night and day improvement in power, shifting (up & down) and even got a consistent 1 mpg improvement.
it's an 87 octane tune so no need to run premium and the videos showing the before and after of an F250/F350 towing a 14k trailer up a slight grade are exactly what I experienced with our 2015 Thor ACE 29.2
The other huge benefit is the change in the shift-points which when in tow-haul mode does such a great job downshifting to control speed downhill, that I almost never had to use the brakes down hill or in town traffic until I needed to come to a complete stop.
I don't know if it applies to your chassis (F53 chassis fix) but the 2nd best mod was the relocation of the sway bar end links to the inner bolt hole position which took ~30 minutes, cost nothing and provided a huge improvement in handling! You can find extensive reviews, pictures ect. under "Cheep Handling Fix" on the IRV2 forum.
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