Did you buy these used or how many hours use caused the buildup?I have 2 gas webastos. I am installing one in a camper I am building and have installed the other in a Pelican case for my ice fishing tent. Can't give you too much input into their function since I've only used it in the tent once so far but it worked well and burnt a minimal amount of fuel for the 6 hours we fished.
They both needed to be disassembled and combustion chambers cleaned out since they were both coked up and throwing codes. After the clean they work like champs! Like you I have a gas truck and motorcycles and didn't want to carry another type of fuel.
I think a little extra caution with installation and refuelling is required due to the fuel type. Overall I feel as safe using the gas webasto as I do driving a gas truck