Calling an electrician

We have a 2014 Itasca with a Norcold 4 door refer. According to the Norcold site, it draws 1.9 amps and needs 108v AC to work. We have a garage with a 100 amp sub panel off the house that I wish to plug the MH's into with a wire buried underground and about 150ft long that will end near the MH's parking pad. The sub panel was installed because the previous homeowner used the garage as a large workshop with many big power eaters. I need power there to operate lights and power doors only, so have much excess capacity.

I am wanting to make sure I use a heavy enough gauge cable so not to have too much voltage drop with I'll say 300-400 watts draw using refer and trickle chargers? 12 or maybe 10 gauge wire? I don't know the formula to calculate voltage drop over 150ft. Thx!