If your goal is to use the fighters as a Spacebourne Warning and Control System affair, then (assuming you're on 6.0) you could design a fighter engine for long endurance flight that would keep speed with your fleet. My fleet's speed is currently standardized at 3333 km/s since that's the first easy ratio I found at Ion tech levels, so I hashed together a quickie fighter, using a Size 2 50% fuel use engine.
XF-120A Siren class Recon Fighter 175 tons 3 Crew 30.4 BP TCS 3.5 TH 12 EM 0
3428 km/s Armour 1-2 Shields 0-0 Sensors 1/1/0/0 Damage Control Rating 0 PPV 0
Maint Life 15.68 Years MSP 11 AFR 2% IFR 0% 1YR 0 5YR 1 Max Repair 14 MSP
Intended Deployment Time: 36 months Spare Berths 0
Pratt & Whitney PROTOTYPE F-12 Long Endurance Drive (1) Power 12 Fuel Use 10.4% Signature 12 Exp 5%
Fuel Capacity 20,000 Litres Range 197.8 billion km (667 days at full power)
RN/APN-14 Fighter Backup Sensor (1) GPS 1400 Range 15.4m km Resolution 100
This design is classed as a Fighter for production, combat and maintenance purposes
It's a tiny smidgeon faster than the standard fleet asset, but that would actually be beneficial in this role as it lets you reposition them easily without your main fleet having to slow down so they can. The sensor's not very big (or good, my sensor tech is kinda crap due to having a 0%, 5 lab scientist there) but given this is a 175 ton fighter what can you expect? If you expanded it to a full Size 1 instead of the Size 0.5, you might have to slap another engine on it or do a slightly less efficient engine design in exchange for a bit more power. But for the purposes of endurance, not sensor range, this design would actually handle it fairly well. And if you need more range, well, you just stick it further out from the fleet really. Almost 2 years of fuel, 15.68 years of maintenance supplies, and crew space enough for three years of deployment... Heck, you could dump these at jump points as a self-mobile sensat.
...You know what, I think I'm going to actually use this thing for that.