With great trepidation I leap into the fray with a slightly different slant on things from Ozarker. There are a couple "styles" of "deep cycle" batts. One is made for cranking AND deep cycle use while the other "True" deep cycle battery is not. The real deep cycle battery will provide a good service life in a deep cycle application and the regular starting battery won't. The deep cycle batt will not provide a satisfactory service life in a starting application if their is an appreciable starting current requirement (which you have).
Yes you can use the deep cyle battery for a starting battery BUT it won't last long and might leave you stranded at an bad time. Its plates and internal busses are not built for high current draw and you will ruin in it short order in all liklihood.
Typically, a deep cycle batt is rated for Amp-hours (AH) only. By convention the AH rating is a 20 hour discharge. For example if a 100 AH battery was set up to deliver 5 amps and lasted 20 hours before discharging to the lower limit voltage prescribed for the test, that would be 100 amp-hours. Before cranking and cold cranking ratings were in common use, amp-hours were specified but as is plain to see that rating is meaningless in a starting application where there is heavy current draw. There is an effect similar to reciprosity failure in film. If you discharged your hypothetical 100 AH batt at a 20 amp rate it would not last 5 hrs but a lot less. The faster you take it out the less you get is the way it works.
Personally I wouldn't think the uncertainty of when the vehicle would strand someone would motivate me to "use up" the deep cycle batt in a vehicle. Buy a starting batt and find another use for the deep cycle: Fence charger, troling motor, emergency power for 12 volt lights or small inverter for use in a power outage, donation to a worth cause, door stop, exercise weight, something but not in a vehicle that you may depend on to not leave you stranded.
Patrick