Primus nailed it with the gears....or
My 03 QC - 3.9 V6 Stick shift, has pulled more weight than your camper, but it was after I installed 3:90 gears and modded the motor.
You don't say what trans you have, but if it's the 3 speed with OD 4th gear, your probably going to be dropping into 3rd to get the rpm to do the job on hills and passing. If you have the 4 speed Auto, make yourself a chart of speed vs RPM, watch your RPM as you drive and have someone write down the RPM in 3rd and in 4th. Then you can see what your engine has to deal with. Seem like the 318 likes ~1500 to 3500 RPM and signs off. This is a curve so if 4th gear has you at 2000-2200 at 65 your dead. (RPM stated is a guess, only you can verify what it's doing).
The other thing that will make a big difference is a modification to the intake manifold to get the RPM your missing. There are pros and cons about it killing the low end torque. I did it to my 3.9 and have been very happy with the result. No exhaust mods, stock exhaust manifolds, runs up to red line if I let it without hesitation. It still has plenty of torque to get started and the biggest factor is the pulling power in mid range right where it fell off prior to the mod.
Do some research on the web site under "kegger mods".
The other two things would be a really good trans cooler and a 180 thermostat.
DrPepper