Do mosfets only output a portion of their drain voltage when measured at their source? Why?
You've built a common drain buffer using FET2 - the characteristic behavior of such buffers is that the source voltage will follow the gate voltage minus a Vgs drop needed to turn on the MOSFET.
The simplest option might be to use a single relay that's rated for enough power - if you want the battery to be disconnected when V_house is present, you can wire V_house to the relay coil and use normally-closed logic for the current path. Make sure the relay can handle the necessary current, especially if there's a chance you might be opening/closing it when operating the starter and/or other loads.
There's also a MOSFET solution that can be used if you absolutely want to avoid relays for some reason:
Assuming that your battery load must be switched at the high-side, if you want fully on-off switching and want to still use MOSFETs, you can use a high-current PMOS instead (note different relative position of source/drain terminals):

(note that I used an extra nMOS inverter rather than simply putting the PMOS directly to V_house, to avoid any potential issues with mismatches between V_house and V_s). I'm also assuming that you're just sensing for V_house being present or not; if you need more precise sensing, you're going to want a comparator and precision reference instead (plus one discrete pFET and some passives).
Some additional schematic notes: Your voltage sources are not drawn as I would expect -- the positive terminal floating is unexpected. You'll want to either draw the supply voltage as a rail (as I drew), or use a source correctly connected to the ground that it's referenced to.