Personally, I'd think that the RORO ferry, once loaded, would be more vulnerable. Then you've got all of the same flammability issues, but now the ship is also at stake and it's harder to fight fires and move not-yet-burning tanker trucks away from a fire.

EDIT:

https://crimeaports.ru/en/affiliates/kerch-ferry-service

Fleet composition: 2 self-propelled ferry

Self-propelled ferry "Yeisk":

The passenger capacity of the "Eysk" c / p is 54 people. (members of the crew of trucks). The maximum possible loading of the ferry with light-duty trucks is 17 units.

Self-propelled ferry "Kerch-2":

The passenger capacity of "Kerch-2" is 60 people. (members of the crew of trucks). The maximum possible loading of the ferry with light-duty trucks is 20 units.

That's also not a whole lot of depth in ferry stock for Russia to be able to afford to lose. It sounds like Ukraine already wiped out Russia's rail ferry capacity to Crimea in the last several months:

https://united24media.com/latest-news/ukraine-knocks-out-russias-last-rail-ferry-slavyanin-supplying-crimea-17733

Ukraine Knocks Out Russia’s Last Rail Ferry “Slavyanin” Supplying Crimea

Apr 08, 2026 13:51

HUR said the ferry had been the last remaining railway ferry used by Russian forces in temporarily occupied Kerch. The vessel was reportedly used to transport fuel, ammunition, military equipment, and weapons to Russian troops stationed on the peninsula.

The intelligence agency noted that the same ferry had previously been targeted in March. At that time, Ukrainian forces also struck another Russian railway ferry, Avangard, which was subsequently disabled.

EDIT2: I guess Russia could also maybe use landing ships to transport fuel trucks onto unimproved shores, if a tractor-trailer can handle sand, but it sounds like Ukraine's been taking them out too:

https://en.wikipedia.org/wiki/Russian_landing_ship_Saratov

She was destroyed on 24 March 2022, while in the port of Berdyansk, after being struck by a Ukrainian missile.[2]

https://en.wikipedia.org/wiki/Russian_landing_ship_Nikolai_Filchenkov

According to Ukraine’s Defense Intelligence, the ship was hit in the night of 19 April 2026 in Sevastopol Bay along with Yamal, and both may have been subsequently taken out of service.[14][15]

https://en.wikipedia.org/wiki/Russian_landing_ship_Nikolai_Vilkov

Based in Vladivostok. I don't expect that it can get to Crimea, since Turkey closed the Turkish Straits to warships subsequent to the invasion.

https://en.wikipedia.org/wiki/Russian_landing_ship_Orsk

Sounds like that one's still in service.