What he said about biocide . Make sure to get it in both tanks and run it through the lines thoroughly . Make sure that you put some in while filling so it will distribute through the tanks, then give each tank a good long time of running so it has lots of time to distribute the stuff completely via the return line and the selector valve gets thoroughly treated. You don't want anything untreated and you want the tanks as full as possible to avoid moisture condensation. If you store a long time you will get a LOT of moisture in your fuel even with the tanks full so do not forget to drain the fitting on your filter very soon after startup a few times
Taking the weight off the tires is good if you can, if not make sure to air them up to max and cover them wiping down with a silicone based tire coating will help preserve them ( don't use armor all ...some so called protectants will actually eat up rubber in the long term... thats the reason a lot of cars off chincy used car lots blow heater and radiator hoses, from being armor alled )
Take your battery connections off and coat with conductive grease to prevent corrosion, best to leave ground cables off to prevent parisitic losses over long term and as mentioned install battery maintainer or those little solar jobs work pretty nicely for long term storage soloutions
Since they won't have current flowing through them, it wouldn't be a bad idea to pop all the fuses out and spray them down with a conductive cleaner/lube so they have a light oil coating on them to resist oxidation. Please do not use dielectric grease, it resists electric current and can cause circuits to burn out.
If it can freeze there, block the wheels and park it in gear but don't set the parking brake, you don't want the thing to get stuck when you do need it
MAke sure to get a tester and check your antifreeze.
