From I-70, west of Idaho Springs, take exit 238 for the Fall River Road/CR 275. This paved county road 275 ends in the vicinity of a mountain village called "Alice," mostly a collection of summer cabins, etc. This is the same road that leads up to the well-known St. Mary's Lake and Glacier. The road follows the Fall River in a generally NW direction. After over six miles miles, the road makes a second, abrupt switchback to leave the Fall River drainage and climb up to St. Mary's Lake and Alice. Continue north into Alice and turn left onto Alice Road. This will take you back west and SW for a short distance. Turn right onto Steuart Raod. This road becomes FR 701.1. This will take you to Loch Lomond. There is a lower and upper gate on this road that are seasonally locked. Access all the way to the Loch according to the Arapaho NF site is generally closed from mid-October to mid- July.
According to one site that rates 4WD roads, FR 701.1 is "suitable for stock, high-clearance 4WD sport utility vehicles. The road is still currently being described as very rough and rocky but easy to handle. Our combined memories recall that the drive back to the Loch was slow enough to make you reconsider if the drive was worth the effort versus hiking by another route. We would say the main advantage of this route is the visit to the upper lakes above the Loch.
On the way back to Loch Lomond on Steuart Road you can find some dispersed, primitive campsites. Just be sure you've driven far enough back to be away from any private property concerns.