That is the correct way to use the LPG system and you are correct in thinking the carb is used for both LPG and petrol. If you remove the air filter box you will see an extra part has been added to the carb body. That is the LPG inlet.
Re-reading your first post I see "when I try to start it, choke out". Does this mean that you are pulling the choke out to start on LPG? The choke is only needed to start on petrol. To start on LPG switch on the ignition for about 1 second, switch off, switch on again for about 1 second and turn to start position. If the engine doesn't fire up repeat the above until it does. When my son started his last week it required four double 'sniffs' of LPG, after two months standing and in freezing conditions.
The previous owner advised me to run it on petrol just for a few minutes every couple of weeks... something about lubrication.
He is correct and my son has pointed out that he does this regularly. If you run constantly on LPG the seals in the carb will dry out causing problems if you need to run on petrol. He does his petrol run for about 10 minutes every other week when he is using the motorhome.
How long is it since a gas service was done? It is recommended that it be done every year. Like anything with an ECU, settings can go off and some LPG systems have filters in the lines which need to be changed. I saw the filters taken out of my son's van after 3 years of use. They were, to be polite, extremely dirty. He had used his van for the 3 years not knowing the LPG system needed to be serviced. After the service he was amazed at the extra power and increased economy on a run that he makes at least four times a year. His annual LPG service is booked for next week before he hits the road for Scotland again.
As a matter of interest for other members who may read this the fuel injected LPG system always starts on petrol and switches to LPG automatically at a preset RPM. My Jeep is set at 1,400rpm. It can, of course, be run totally on petrol but the changeover switch only has two positions as opposed to three on the carb version.
Work was good - retirement is better.
1996 850GLT 2.5 20v Estate Manual.
1995 Peugeot Boxer 2.5Tdi Autosleeper.
1984 244DL, Manual, Beige.
1987 744GLE, Manual, Green.
1991 960 3.0 24v, Auto, Silver.
1994 940T Wentworth, Auto, Blue.