Colorado camper rental is good for trailers, but they are likely mostly if not completely booked up by now. We rented a trailer from them several years ago before we decided to buy and found they we're booked until after Labor Day, which is when ended up going. They are very good people and spent a lot of time going through the rig before we left.
You also can rent from Lazy Daze in Loveland - trailers, Class A, Class C, Class B or from El Monte RV in Saldalia (near Castle Rock) - Class A or Class C
https://www.lazydays.com/rentals?callid=1&cid=96-0008-01&atrkid=V3ADW8CB877AF_40739245844_kwd-98148736__258354351581_g_c___1t1&trkid=V3ADW493584_40739245844_kwd-98148736__258354351581_g_c__&gclid=EAIaIQobChMI7--lp9LY2wIVGbbACh1UwwVuEAAYASAAEgL2RPD_BwE
https://www.elmonterv.com/rv-rental/where-you-can-rent-rvs/rv-rentals-location-denver-rv-rentals/?gclid=EAIaIQobChMI9IPR_9HY2wIVAaZpCh1H0Qn-EAAYASAAEgI0YvD_BwE