A fusion-powered vehicle can produce hydrogen fuel every day as long as it has access to water. It can also provide the power for a field base, recharge local batteries, etc.
So depending on fuel usage of accompanying vehicles, a fusion-powered vehicle supporting the rest of the logistics train can be fairly independent of supply lines.
From Campaign Operations page 202 (MSR&C -> Repair & Replacement -> Obtaining Replacement Parts -> Fuel) a solar, fission, or fusion-powered vehicle can produce 10 tons of fuel per Maintenance/Repair Cycle if local fluid is available. From page 188 this is a time period of 8 hours.
Even an infantry unit can benefit. Campaign Operations page 24 (Stage 4 Determine Operating Expenses -> Step 1 Complete the TO&E -> Ammunition -> 3rd paragraph -> 4th sentence): if the infantry are armed with non-plasma energy weapons their ammunition costs are divided by 10 as the reactor can provide the power and only worn/damaged power packs need replacing.
(I basically searched for 'fission' in Campaign Operations, Strategic Operations, and Tactical Operations, and selected the ones that listed how much hydrogen fuel was generated. I didn't want to search for 'fusion' in those books and have to click 'next' over and over. I do remember a reference to a fission or fusion plant producing a quarter or half ton of fuel per day, but don't remember where that is from off-hand.)
The key problem is disguising it as something boring so the enemy is less likely to shoot at it. One idea would be to make it identical to the supply vehicles your logistics train already has in large numbers. The key problem would be figuring out a way to disguise the heavier power cables coming from it. A water supply to be converted into hydrogen fuel would be handled by an onboard tank of water so the water only has to be transferred once per day (vs power being needed 24/7 when the logistics train is stopped).
You will need the specialists to handle the fusion reactor, and that is where your primary issues might happen, making sure your specialists don't decide to retire and get a comfy civilian job. One idea could be mandatory longer service terms, or a bonus to be paid out only if they complete their initial service term, or higher paychecks for critical skills, or whatever other option would work.
It also means you expect to be using this vehicle often enough. If the planet isn't raided that often then ICE vehicles with pre-staged fuel depots might be practical. But if you expect to be away from bases and still need fuel/energy supplies then a fusion-powered vehicle would certainly help. Heck, even a
fission-powered vehicle might work.
Perhaps a Disaster Aid vehicle? A hurricane hits a city and takes out the incoming power grid, so you need help. This vehicle arrives first, hooks up to the local power grid, and now you have a power supply. Not as many people are going to be shooting at it, it will be surrounded by people keeping an eye on it, and it will keep on providing power off a very small fuel supply. As an example, Miami uses ~1.2 GW. You'd have the vehicle pop out first, and activate the local spaceport and nearby hospitals. The spaceport cleans itself up and a Dropship then arrives to power the rest of the city. The spaceport would likely have power connections to the main power grid for just such an occasion.
