Agree with the above, usually works a dream.
That said, here are some other ideas. First thing I would do is check the mags or electronic ignition - check timing, check impulse etc. Make sure you have ignition available at the correct time.
The solution starts with a question - how long has the engine been off ?
I am very familiar with IO-360, IO-540 and Continental IO-520 and can add a little for each.
Pump off, throttle cracked a tad - enough so that it is where you went from idle to say 1,000 rpm. Try it when running and see where the throttle is. Mixture ICO. Crank until first sign of firing then smoothly bring mixture to full rich, adjust throttle and be ready to run the pump if stumbling occurs. This is because our RV's are tightly cowled and retain heat so there may be bubbles of vapour in the firewall forward.
This is the interesting area usually as the motor doesn't know what it is doing ! You need to know what fuel system you have - is it direct to fuel servo or does it have a return line to the tank ?
A. No return line - try method above. If no success, mixture rich, throttle part open, run the pump for 2 - 3 seconds only. Back to start, mixture ICO, throttle cracked, crank and smoothly bring mixture out of ICO upon first sign of firing, be ready for fuel pump to keep it running smoothly.
B. Return line - Keep mixture in ICO and throttle open a way. Run fuel pump for 5 seconds to purge the line of any vapour. Back to start, mixture ICO etc etc.
After that you are out of reasons why a motor like that won't start unless there is something lurking in the induction system or ignition system