A 180Ah lead-acid battery like the Varta Professional LFD180 only delivers about 90Ah usable at the 50 percent depth of discharge that keeps lead healthy, roughly 1,080 Wh. Because LiFePO4 safely uses about 80 percent of its capacity, you need only around 106Ah of lithium to match it, at a fraction of the weight.
Best-value lithium replacement
Power Queen 12V 200Ah PLUS
Gives 2,048 Wh usable against about 1,080 Wh from your lead, and saves roughly 32 kg.
Fit: your Group 4D box is 527x222x250 mm, the lithium is 522x240x220 mm. Check the space, larger than one original box
Renogy 12V 300Ah (Self-Heating)
Before you swap
A lithium battery that fits the tray is not automatically plug and play. Check your charger profile, fit a DC-DC charger if you charge from an alternator, set any solar MPPT to lithium, review fusing and cable, add a shunt monitor, and never mix lithium with lead. For your setup see the marine, campervan and off-grid guides.