What brand(s) of tow vehicle are you running? Do they not have a service manual specific for this kind of use? Typically, converted chassis-cab type trucks get unique service manuals. I would think an hour based maintenance program would be good for you.
You can always do an "guesstimate" of your equivalent by perhaps guessing an average speed of 15-20mph, then converting hours into a mileage mark. I.E. say you average 20mph; you want to OCI the equivalent of every 5k miles. Your OCI would be set for 250 hours. You can play with the numbers as you see fit for your comfort. Of course, this depends if the veh has an hour meter, but you could install one as they are not expensive.
Your other choice would be UOAs. Perhaps UOA every 200 hours and track oil and engine condition. Why guess when you can know?!?!
I personally have some off-road stuff that goes strictly by hours; Kubota and Scag both have hour meters on them, and obviously no odometer. Very common for non-road stuff to run this way.
My Duramax equipped truck has an odo and hour meter. I seem to average about 37mph overall for several years of ownership now. This veh is used for about 50/50 city/hwy. This is my basis for stating you may want to make your estimate around 15mph or maybe no more than 20mph as an equivalent.