I have a 27ft Itasca w/ a chev 454, throttle body fuel injection. Took it out for our first long trip over mutliple passes. Fully loaded with boat in tow. The engine runs fine on the flats but when I start to climb and open the throttle up it will bog down then backfire through the throttle body. If I back off a bit it will eccellerate and go ok till I give it more throttle. It feels like its at the point were the secondarys would kick in if it had them, right before it should down shift. I'm thinking its a fuel delivery issue so I changed the fuel filter. There is one in the middle of the chassi with both endes threaded. I was told by a mechanic that that is the only one. I also cleaned the contacts in the cap/rotor (they are new). It has new plugs and new high heat wires. Checked the timing. It was advanced to about 10 degrees so I set it back to 4/6. Thought all of this would fix it but it didn't. I need help. I have planned another week long trip in about two weeks so I gotta get it fixed. thanks for all the help.