hey doug, congrats on building a van. figuring out your 12 volt power systems can be challenging when you are first learning. i'll see if i can help you here a bit.
so the short answer to you solenoid adequacy question is, maybe? like someone posted above. we would need more info to give a definitive answer. i will try to explain it so you can understand and then make the decision based on your particulars
first let look at your question on the "continuous duty solenoid". simply put, the solenoid is just a big switch the is turned on/off by electricity instead of you having to flip a switch with your hand. when looking at the amp ratings, 85 amp in your case. you could have either a continuous rated solenoid or an intermittent rated solenoid. an example of an intermittent solenoid would be in the starting circuit of your van. when you turn the key to crank the engine. they key switch sends power to the starter solenoid (either built into the starter, or external mounted separately) which closes allowing the high current to get from the battery to the starter. this can be a couple hundred amps. but it only lasts for a few seconds. that type wont last long at all if you use it for more than in short spurts. that is where the continuous duty type comes into play. the continuous rated one is meant to be "on" for extended periods of time, i.e. continuously...
the purpose of this solenoid is to connect the starting battery to the house battery and allow them to share the output of the engine alternator for charging. by wiring it the way they show in the video lets it energize or turn on when you turn the ignition on, and turn off when the ignition is turned off. it is not fool proof, but that basically allows the house battery to charge from the engine but not discharge the starting battery so you dont end up with a dead starting battery and can still start your van when you want
now, to decide if your 85 amp rated (assuming continuous duty rated) solenoid is big enough for your application. we have to look at how many amps might be going through it and on to charge the house battery. from a purely technical by the spec way to figure that would be to check the output rating of your alternator. if your solenoid rating was higher than the output of the alternator you would be good to go (except for a couple rare situations where you are also drawing power from the starter battery, only mention for those that want to nit pic)
now your 85 amp rated solenoid may be just fine, even if the alternator is larger. say you have a 130 amp alternator. it is unfortunate, but they rarely put out full capacity and even then they tend to taper of rather quick. they also are sharing that out put between 2 batteries. so the amps going through the solenoid will hardly ever approach the full out put of the alternator. your house battery would have to be really really low at the same time your starting battery was full (so none of the alternator output was going there) it just doesnt happen very often. for example, i had a 40 amp fuse protecting my "solenoid" and even with a 95 amp alternator i only blew that fuse 2 times in 3 years and over 50,000 miles. so if you protect the solenoid with an 80-85 amp fuse you will be fine. the fuses in that size can be a little pricey so i would recommend getting an 80 amp breaker. that way you just have to reset it if you pop it. you can learn how to not pop it by being aware of the condition of your system. it is only going to pop when the house batteries are real low and drawing a lot of charge current. if the breaker pops, before resetting the breaker, leave the engine running at idle. (at idle the alternator wont be able to put out max current) also turn on the headlights and high beams and the fan for your heater. you are trying to use enough amps from the alternator that the remaining output going to through the solenoid is less than the breaker. then reset the breaker, let the engine run a few min at idle like that and then when you start driving leave the lights/heater and whatever else on for a bit till the house bank charges up some and no longer drawing such a large charge current.
now that we are talking about circuit protection and amp rating you need to understand how to choose wire size in relation to the fuse or breaker size. fuses and breakers are there as the week link. they are meant to fail or trip before a dangerous over current situation can cause damage or start a fire. in this case here we selected the rating of the breaker to protect the solenoid. we now have to make sure the wire/cable we use is safe to handle the full rated current of the fuse/breaker. there are lots of charts you can find on google. look for wire size fuse chart or wire size amp capacity. for safety, i.e. to prevent melted wires and fire that will be around an 8 or 6 gauge wire, minimum! this would be a safe size. but you probably want to use a larger size to reduce resistance and get more of the output actually to the house battery. you see longer wires have more resistance and cause a voltage drop. this is just wasted energy. you want to use an online voltage drop calculator. i like this one...
https://www.calculator.net/voltage-drop-calculator.html i personally wont install a with more than a 3% voltage drop and strive to get to 1% but you will have to weigh the extra cost of the heavier gauge wire.
