Stunned any Fronty can approach 3-tons! Wow! My 2009 King Cab is listed 3600 but scales 4000. 2WD 4-cyl 5-speed manual.
First, IS yours flat towable? I've looked at this a lot, and conclude that ONLY manuals can be flat towed. 2WD in N, 4WD in 2H and N. Automatics NOT towable and since Transfer Case has no Neutral 4WD is no help over 2WD, which used to almost always be the fix. Better check from Owner Manual, not Salesman, and study the details.
OK, It's 4x4, manual shift, good to go.
Is E450 rated for it? YES. GCWR is 22000, GVWR is 14500 so fully loaded RV can tow 7500.
Is FRAME rated to tow 7500? YES. Chassis Stretchers must comply with a Ford Standard (short name FMVSS) that requires extended to match original specs.
Is HITCH rated to tow 7500? MAYBE, look. Many late model C's on E450 are shipping with a 7500/8000 hitch. You can upgrade.
Will I feel it? HELL YES. Lighter the Easier, Heavier the Harder. You'll feel holdback on acceleration and grades. Probably will no longer pass big rigs on mountain upgrades. Might be with them in that Truck Lane. BUT!!! You CAN unhitch and drive separately for a few hours through hard areas.
Will it damage the E450? NO!!! And I said NO. It's a rugged commercial duty BEAST built to do it. Ford Tough and that's no joke.
Fuel Economy? FORGET that word, just say MPG and you'll be lucky to get into 8 on level ground. Mountains? This is only a guess: 5-6. GAS is still the cheapest part of the RV Experience.
About 7 pulling Fronty but I've seen 8 on long trips. Better MPG in hills with cruise OFF. If I can I run 60-65 off cruise in hills. Cruise at 62.5 on flats.
Driving 55 produced WORSE mpg as does 70.
I also think Fronty de-stabilizes tracking a little, so when eBay had a used base plate for wifes 2006 Corolla, I bagged it. Now we don't take Fronty unless we need him. Just easier.
Finally what are your expectations? V10 sounds like a monster. Zippy in E250. Cram one in a Mustang and it's nearly a Viper. In 20,000 of Class C + Fronty it won't turn any heads. El Bajada from 5000 foot ABQ toward 7000 foot Santa Fe on I-25 Northbound it will be a SLUG. But it won't overheat, it won't burn the transmission out, and you, your family, your motorhome and your Fronty will all be in beautiful high desert ready to stay and play.
Good Luck! Enjoy!