Gary's right if you want to go full depth analysis of your gear and the way any decent Ham or RF tech would go.
You can go shelling out for dummy loads and RF wattmeters, use them once, never have a use for them again and ultimately be disappointed with what you find......
The first thing you'll find is that your linears are not pushing anywhere near 300 watts....
I won't get into the whoop-ass arguments about P.E.P (peak envelope power) or other such nonsense.
You are getting much more power out than you put in so be happy with that
The front switch labelled 1 to 6 is a simple resistor attenuator control to limit how much power you put into the linear, 1 being the most attenuation, 6 being straight through. It's "sort of" a power control.
(Those resistors can get really hot too, I've seen them blackened to hell!)
It's pretty much the same job as your RP2 is doing. You ain't ever going to figure it out, just accept 1 is the lowest and 6 is the highest.
Heat - you're gonna get heat.
Those front end resistors are getting hot and so are the transistors and that heat has to go somewhere. You just gotta hope the heatsink has a good enough airflow to take that heat away. As Gary has again mentioned, people put fans on them to help draw that heat away.
(Again , I won't get into the poor efficiencies of those transistors, the push-pull circuitry or losses through the class B biasing!)
Put it this way, my rig has a similar circuit of those linears built right into the radio....most of the radio casing is a big-ass heatsink, with ducting and a fan....designed for Hams. Hams are notorious for rag chew long overs
You're also right, they're gonna smell a bit funny to start with just a like a new oven does.
I'd run the RP2 at the quarter setting and the linears nearer 6 for best results. You should be able to chew the rag all day long.
(Don't do the trucker trick of running them at 24 volt - you'll get more power, just briefly before they blow up!)
As for wiring to the car, Gary again is right, (he is a helpful bloke)
DON'T use the ciggie lighter socket! You have been warned.
My rig at full power runs some 19 amps, your homebase is running on a 30 amp pack, figure for direct to battery wiring along those lines. Most guys run this sort of wiring for their big-ass music systems so there's plenty of info out there.
Just a word of warning - I've known guys park on top of a hill, run their linears on a night time DX or net, pack up for the night and the car won't start........flat battery!! Take a spare or jump pack with you just in case
Sheesh, waffled as long as you...hee, hee
Anyway, hope that's of some help.
All the best,