You said you ran 2awg cables, plural, under the van. That makes me think instead of attaching the lifeline - terminal to nearby frame as many would do, you ran a black cable all the way to engine battery, physically in parallel wth the red cable?
If so, I approve as frame grounds seem to become problematic all too soon.
Your blue seas relay is a fine unit, my only issue with it is that when either battery sees charging current, from any source, it will combine the battery banks. Sounds fine and dandy, but if the solar is holding the lifeline above the voltage setpoint of the Blue seas ACR. then the engine battery will be sucking up some of the solar wattage, rather than all of it feeding the depleted lifeline house battery. The engine battery might not require being held at these higher voltages and could be overcharged. it depends on whether you also slightly cycle the engine starting battery, which do not really like being cycled.
Your stated loads are quite small. Your system is quite overbuilt if your loads are not going to increase rather dramatically, or if you go days on end with no solar input.
As far as the 51 amps minimum, that is when the battery is regularly depleted to the 50% or below range. Lifeline batteries basically have no upper amperage limit, they can handle whatever, as long as the voltage does not get too high( 14.4v at 77F). A 8d lifeline at 50% state of charge can likely accept well over 200 amps for at least 20 minutes before the voltage at the battery terminals rises to 14.4v, so do not fear overamping this battery. Overvolting might be a brief possibility with the alternator's voltage regulator but really i do not know what fords do in this regard.
IF you were trying to max out the alternator contribution to depleted lifeline, you could either move the 2awg cable from engine battery to + alternator output stud, and the - to an alternator mounting bracket. This has the advantage of being able to eliminate one of the fuses. Fuses are supposed to be close to the battery terminal, and the original alternator charge cable to engine battery is likely already fused, so you do not need one in between alternator and ACR, but you do need one between ACR and Lifeline if you choose to move the cables.
If you choose to NOT run the 2awg cables to alternator, then the original alternator to engine battery charging circuit now will have to pass the charging current for the lifeline, and it was not intended to do this. It is likely much tinner than 2awg. So you can add another cable between alternator (+) output stud and the engine battery to remove this bottle neck.
If you have grounded the lifeline to the engine battery, the original engine to engine battery ground is going to also carry the lifeline carging current, and it too is likely undersized for the extra 50 amp load, and will cause a resistive electrical bottle neck, slowing down charging. You can do the same, either add another parallel cable, or replace the original cable with a thicker one, or both if you really want to go crazy.
But with the loads you have stated I think the extra maximizing of charge current steps are not needed. With the ACR being bidirectional, the solar will be recharging the engine battery, so self jumpstarting is extremely unlikely to ever be required. If it is, do as recommended previously. Press the manual connect, wait a minute or longer for the lifeline to feed/charge the depleted engine battery, and it is very unlikely the 140 amp fuse would blow when starting the engine.
Lifeline has a very good manual on what their batteries would like to maximize their performance and longevity.
http://lifelinebatteries.com/wp-content/uploads/2015/12/6-0101-Rev-E-Lifeline-Technical-Manual.pdfRolls surrette also have a very well written manual that can help a Newb to lead acid battery living understand what batteries require to perform well and last a long time.
http://rollsbattery.com/public/docs/user_manual/Rolls_Battery_Manual.pdfIf you were to say, add a microwave and a 2KW inverter to power it and nuke 3 meals a day, then you would want to maximize alternator contribution and perhaps even run engine and microwave at same time, but the ARB fridge, and some USB ports are not going to put much of a dent in the 8d lifeline.
Read the Lifeline and rolls manuals more than once, slowly.
