I'd try and attitude adjustment first. you're towing a heavy object driving a heavy object. If you can rationalize being OK with slowing down a little bit, while also visualizing the cash saved (and thus available for other purposes) you'd be golden.
Otherwise ... tough call to throw more money at your current rig. I have a 1999 F150 and I go through this every now and then, for now I haven't messed with it. Newer trucks have more powerful engines but more importantly 6 or 8sp transmissions to work with. An 8 speed transmission would make a huge difference just on its own.