54K miles on E450 V10 is moderate and not concerning. V10 engines commonly exceed 200K miles. Check maintenance records and get pre-purchase inspection focusing on overall condition.
The 6R140 in F53 and F550 is fundamentally the same transmission. Physical parts interchange. Differences are in electronic calibration and torque converter selection for each application. For repairs or replacement, they are compatible.
Tow/Haul mode works with cruise control. Use Tow/Haul for all RV driving - better shift points and engine braking.