I have a Newel coach RV with a 15 ltr. Caterpillar diesel engine. Now with two AGM batteries 8DL 12V each 255A to start the engine. Would it be possible to use one LiFePo4 battery 300A to start this engine?
TL DR: In a word, No
You are mixing up terms here. The battery you are talking about is a GPL-8DL 12vdc battery. It has a 255 Amp Hour rating. Your battery pack has 300 Cranking Amps of power. These are completely two different things. This example of your battery has 1350CCA (Cold Cranking Amps) at 0°F ... then there are two of these, which are most likely ran in series to give you 24vdc. Your little LiFePo4 battery pack won't even touch the needs of the Caterpillar diesel engine. You'd need to get 10 of these packs (five each ran in parallel, then the two five packs ran in series), fully charged, to even come close to doing what you'd need to do. Then, they'd most likely discharge too quickly to actually get the engine started.